the protocol for this cross - sectional study was reviewed and approved by the institutional ethical committee for clinical research at the geneva university hospitals . all patients who had received a whole pancreas or an islet of langerhans transplant at the geneva university hospitals and who had regular follow - ups at our institution were asked to participate in the study . patients were required to have at least partial graft function ( as determined by positive c - peptide levels ) . fifteen islet transplant recipients were enrolled in the islet group , which was further subdivided into an insulin - independent group ( ii - isl ) and an insulin - requiring group ( ir - isl ) . ten healthy nondiabetic volunteers were recruited to serve as control subjects . finally , five patients with type 1 diabetes awaiting pancreas or islet transplant , all of whom demonstrated no c - peptide production at the time of study inclusion , served as negative control subjects . written informed consent was obtained from all patients and control subjects prior to their participation in the study . demographics and population characteristics all samples were obtained from patients during either their annual posttransplantation follow - up or one of their regularly scheduled outpatient consultations . the serum was labeled using the same anonymous codes assigned to the tubes at the time of the test . an arginine stimulation test was performed on all subjects to determine basal and stimulated pancreatic hormone levels . blood was drawn at 10 , 0 , 2 , 3 , 4 , 5 , 7 , and 10 min from intravenous injection of a 5 g arginine bolus . patients who were back on insulin were required to discontinue slow - acting insulin at least 24 h prior to the test and rapid - acting insulin at least 6 h before the test . serum levels of insulin , c - peptide , total proinsulin , and intact proinsulin ( ipi ) were then measured using commercially available elisas as described below . acute insulin response ( air ) was calculated as the mean of the three highest values between 2 and 5 min minus the mean of the basal values at 10 and 0 min . the area under the curve ( auc ) between 0 and 10 min postinjection for insulin and c - peptide was calculated by the trapezoidal rule with the mean of the baseline values subtracted . proinsulin fragments , including split-32,33 proinsulin , des-31,32 proinsulin , split-65,66 proinsulin , and des-64,65 proinsulin , were obtained by subtracting ipi from total proinsulin . ratios of insulin to total proinsulin and insulin [ insulin/(total proinsulin+insulin ) ] as well as proinsulin fragments to total proinsulin ( proinsulin fragments / total proinsulin ) were calculated to quantify the rate of proinsulin processing . ratios representing peak stimulation were calculated as the mean of ratios for the three highest time points between 2 and 5 min after arginine stimulation . insulin/(total proinsulin+insulin ) ratio represents the quantity of fully processed insulin with respect to all circulating forms of the hormone . commercially available elisas were used to measure hormone levels for insulin , c - peptide , total proinsulin , and ipi . the following kits were used : insulin ( mercodia , uppsala , sweden ) , c - peptide ( mercodia ) , total proinsulin ( millipore , zug , switzerland ) , and ipi ( millipore ) . concentrations were determined using a standard curve with high- and low - level control subjects . cross - reactivities of the assays were provided by the manufacturer and are as follows : the insulin assay cross - reacts 98% with des-64,65 proinsulin , < 0.01% with c - peptide and proinsulin , and < 0.05% with des-31,32 proinsulin and split-32,33 proinsulin ; the c - peptide elisa cross - reacts < 0.0006% with insulin , < 1.8% with proinsulin , 10% with split-65,66 proinsulin , 74% with des-64,65 proinsulin , 3% with des-31,32 proinsulin , and 2% with split-32,33 proinsulin ; the total proinsulin elisa cross - reacts 100% with intact and des-31,32 proinsulin , and 81% with des-64,65 proinsulin ; the ipi elisa cross - reacts 36% with des-64,54 proinsulin , and it shows no cross - reactivity with des-31,32 proinsulin . the lower limits of detection of the assays were provided by the manufacturer and are as follows : the insulin assay has a detection limit of 6 pmol / l , the total proinsulin assay has a limit of 0.5 pmol / l , and the ipi assay has a lower limit of 0.1 pmol / l . all statistical analyses were performed using prism 6 software ( graphpad software , la jolla , ca ) . for all analyses , values obtained from healthy subjects were used as control subjects . for data that were normally distributed , student t test was used to compare two groups , and one - way anova was used to compare between more than two groups . for data that had non - gaussian distributions , we used the mann - whitney test to compare between two groups and the kruskal - wallis test to compare between more than two groups . all samples were obtained from patients during either their annual posttransplantation follow - up or one of their regularly scheduled outpatient consultations . the serum was labeled using the same anonymous codes assigned to the tubes at the time of the test . an arginine stimulation test was performed on all subjects to determine basal and stimulated pancreatic hormone levels . blood was drawn at 10 , 0 , 2 , 3 , 4 , 5 , 7 , and 10 min from intravenous injection of a 5 g arginine bolus . patients who were back on insulin were required to discontinue slow - acting insulin at least 24 h prior to the test and rapid - acting insulin at least 6 h before the test . serum levels of insulin , c - peptide , total proinsulin , and intact proinsulin ( ipi ) were then measured using commercially available elisas as described below . acute insulin response ( air ) was calculated as the mean of the three highest values between 2 and 5 min minus the mean of the basal values at 10 and 0 min . the area under the curve ( auc ) between 0 and 10 min postinjection for insulin and c - peptide was calculated by the trapezoidal rule with the mean of the baseline values subtracted . proinsulin fragments , including split-32,33 proinsulin , des-31,32 proinsulin , split-65,66 proinsulin , and des-64,65 proinsulin , were obtained by subtracting ipi from total proinsulin . ratios of insulin to total proinsulin and insulin [ insulin/(total proinsulin+insulin ) ] as well as proinsulin fragments to total proinsulin ( proinsulin fragments / total proinsulin ) were calculated to quantify the rate of proinsulin processing . ratios representing peak stimulation were calculated as the mean of ratios for the three highest time points between 2 and 5 min after arginine stimulation . insulin/(total proinsulin+insulin ) ratio represents the quantity of fully processed insulin with respect to all circulating forms of the hormone . commercially available elisas were used to measure hormone levels for insulin , c - peptide , total proinsulin , and ipi . the following kits were used : insulin ( mercodia , uppsala , sweden ) , c - peptide ( mercodia ) , total proinsulin ( millipore , zug , switzerland ) , and ipi ( millipore ) . concentrations were determined using a standard curve with high- and low - level control subjects . cross - reactivities of the assays were provided by the manufacturer and are as follows : the insulin assay cross - reacts 98% with des-64,65 proinsulin , < 0.01% with c - peptide and proinsulin , and < 0.05% with des-31,32 proinsulin and split-32,33 proinsulin ; the c - peptide elisa cross - reacts < 0.0006% with insulin , < 1.8% with proinsulin , 10% with split-65,66 proinsulin , 74% with des-64,65 proinsulin , 3% with des-31,32 proinsulin , and 2% with split-32,33 proinsulin ; the total proinsulin elisa cross - reacts 100% with intact and des-31,32 proinsulin , and 81% with des-64,65 proinsulin ; the ipi elisa cross - reacts 36% with des-64,54 proinsulin , and it shows no cross - reactivity with des-31,32 proinsulin . the lower limits of detection of the assays were provided by the manufacturer and are as follows : the insulin assay has a detection limit of 6 pmol / l , the c - peptide elisa has a limit of 15 pmol / l , the total proinsulin assay has a limit of 0.5 pmol / l , and the ipi assay has a lower limit of 0.1 pmol / l . all statistical analyses were performed using prism 6 software ( graphpad software , la jolla , ca ) . for all analyses , values obtained from healthy subjects were used as control subjects . for data that were normally distributed , student t test was used to compare two groups , and one - way anova was used to compare between more than two groups . for data that had non - gaussian distributions , we used the mann - whitney test to compare between two groups and the kruskal - wallis test to compare between more than two groups . subjects in the healthy control group were slightly younger than the rest of the study population ( p < 0.01 ) . islet recipients exhibited higher fasting glycemia and hba1c levels than other groups ( p < 0.05 ) . nonetheless , hba1c levels remained in the normal range for the ii - isl group . creatinine levels were elevated in the pancreas and ir - isl groups with respect to the ii - isl and healthy control groups ( p < 0.005 vs. control , p < 0.05 vs. ii - isl ) . figure 1a and b shows the insulin and c - peptide response curves for all five groups . the insulin and c - peptide response curves maintain the same overall shape in all groups , with the exception of the diabetic control subjects who demonstrate no response to arginine stimulation . insulin levels were higher in pancreas recipients but lower in islet recipients compared with healthy control subjects . insulin levels in the pancreas group were significantly higher than those of the healthy control group . airs for healthy control subjects , ir - isl , ii - isl , and pancreas groups were 217  163 , 23  13 , 125  90 , and 266 the c - peptide acute responses in the same groups were 717  362 , 91  53 , 278  124 , and 550  244 pmol / l , respectively ( supplementary table 1 ) . mean acute c - peptide responses were similar in the healthy control group and pancreas group but significantly lower in the islet groups . the ir - isl transplantees showed a significantly decreased acute insulin and c - peptide response with respect to all other groups . there was no significant difference in air between the ii - isl and control groups . as expected , the diabetic control group showed negligible insulin and c - peptide levels . a : mean insulin levels during arginine stimulation of healthy control group , ir - isl group , ii - isl group , pancreas group , and diabetic control group . b : mean c - peptide levels during arginine stimulation of healthy control group , ir - isl group , ii - isl group , pancreas group , and diabetic control group . auc values demonstrated similar distributions to acute responses in the various study groups ( supplementary table 1 ) . auc values for insulin response in healthy control subjects , ir - isl , ii - isl , and pancreas groups were 1,332  959 , 152  91 , 774  528 , and 1,540  707 pmol these values were higher in pancreas recipients and lower in islet recipients compared with healthy control subjects . auc values for c - peptide response in healthy control subjects , ir - isl , ii - isl , and pancreas groups were 4,955  3,158 , 630  428 , 1,843  875 , and 3,859  2,061 pmol the auc of c - peptide for the ir - isl group was significantly lower than in all other groups . mean values for c - peptide and total proinsulin before and after arginine stimulation are shown in fig . these figures show that total proinsulin levels are reduced in the ir - isl group with respect to all other groups and follow a distribution similar to that of c - peptide . a : box plots of basal c - peptide and total proinsulin levels in healthy control group , ir - isl group , ii - isr group , and pancreas group before arginine injection ( basal values ) . b : box plots of stimulated c - peptide and total proinsulin levels in healthy control group , ir - isl group , ii - isl group , and pancreas group during peak stimulation after arginine injection ( stimulated values ) . data are expressed as median ( solid line ) , interquartile range ( box ) , and range ( whiskers ) . the healthy control group demonstrated lower basal proinsulin - processing rates than the pancreas ( p < 0.01 ) and ii - isl ( p < 0.05 ) groups [ 74  14% vs. 83  7% vs. 86  12% , respectively , for insulin/(total proinsulin+insulin ) and 54  25% vs. 67 18% vs. 76  16% , respectively , for proinsulin fragments / total proinsulin ] . the ir - isl group displayed a basal insulin/(total proinsulin+insulin ) rate of 75  14% and a basal proinsulin fragments / total proinsulin rate of 54  25% . a : box plots of the proinsulin - processing ratios [ ( insulin / total proinsulin+insulin ) and ( proinsulin fragments / total proinsulin ) ] in healthy control group , ir - isl group , ii - isl group , and pancreas group before arginine injection ( basal values ) . b : box plots of the proinsulin - processing ratios [ ( insulin / total proinsulin+insulin ) and ( proinsulin fragments / total proinsulin ) ] in healthy control group , ir - isl group , ii - isl group , and pancreas group during peak stimulation after arginine injection ( stimulated values ) . data are expressed as median ( solid line ) , interquartile range ( box ) , and range ( whiskers ) . i / tp+i , insulin / total proinsulin+insulin ; pf / tp , proinsulin fragments / total proinsulin . 3b . interestingly , healthy control subjects , pancreas recipients , and ii - isl recipients all attain similarly elevated processing ratios after stimulation ( 93  2% vs. 93 3% vs. 93  5% , respectively , for insulin/(total proinsulin+insulin ) and 80  7% vs. 77 9% vs. 76  12% , respectively , for proinsulin fragments / total proinsulin ) , while ir - isl transplant recipients ratios do not demonstrate a comparable increase . although ir - isl proinsulin - processing ratios do increase slightly after stimulation to 87  4% and 58  18% for insulin/(total proinsulin + insulin ) and proinsulin fragments / total proinsulin , respectively , these values remain significantly lower than all other groups ( p < 0.01 vs. control and pancreas groups , p < 0.05 vs. ii - isl ) . subjects in the healthy control group were slightly younger than the rest of the study population ( p < 0.01 ) . islet recipients exhibited higher fasting glycemia and hba1c levels than other groups ( p < 0.05 ) . nonetheless , hba1c levels remained in the normal range for the ii - isl group . creatinine levels were elevated in the pancreas and ir - isl groups with respect to the ii - isl and healthy control groups ( p < 0.005 vs. control , p < 0.05 vs. ii - isl ) . figure 1a and b shows the insulin and c - peptide response curves for all five groups . the insulin and c - peptide response curves maintain the same overall shape in all groups , with the exception of the diabetic control subjects who demonstrate no response to arginine stimulation . insulin levels were higher in pancreas recipients but lower in islet recipients compared with healthy control subjects . insulin levels in the pancreas group were significantly higher than those of the healthy control group . airs for healthy control subjects , ir - isl , ii - isl , and pancreas groups were 217  163 , 23  13 , 125  90 , and 266  129 pmol / l , respectively . the c - peptide acute responses in the same groups were 717  362 , 91  53 , 278  124 , and 550  244 pmol / l , respectively ( supplementary table 1 ) . mean acute c - peptide responses were similar in the healthy control group and pancreas group but significantly lower in the islet groups . the ir - isl transplantees showed a significantly decreased acute insulin and c - peptide response with respect to all other groups . there was no significant difference in air between the ii - isl and control groups . as expected , the diabetic control group showed negligible insulin and c - peptide levels . a : mean insulin levels during arginine stimulation of healthy control group , ir - isl group , ii - isl group , pancreas group , and diabetic control group . b : mean c - peptide levels during arginine stimulation of healthy control group , ir - isl group , ii - isl group , pancreas group , and diabetic control group auc values demonstrated similar distributions to acute responses in the various study groups ( supplementary table 1 ) . auc values for insulin response in healthy control subjects , ir - isl , ii - isl , and pancreas groups were 1,332  959 , 152  91 , 774  528 , and 1,540  707 pmol these values were higher in pancreas recipients and lower in islet recipients compared with healthy control subjects . auc values for c - peptide response in healthy control subjects , ir - isl , ii - isl , and pancreas groups were 4,955  3,158 , 630  428 , 1,843  875 , and 3,859  2,061 pmol the auc of c - peptide for the ir - isl group was significantly lower than in all other groups . mean values for c - peptide and total proinsulin before and after arginine stimulation are shown in fig . these figures show that total proinsulin levels are reduced in the ir - isl group with respect to all other groups and follow a distribution similar to that of c - peptide . a : box plots of basal c - peptide and total proinsulin levels in healthy control group , ir - isl group , ii - isr group , and pancreas group before arginine injection ( basal values ) . b : box plots of stimulated c - peptide and total proinsulin levels in healthy control group , ir - isl group , ii - isl group , and pancreas group during peak stimulation after arginine injection ( stimulated values ) . data are expressed as median ( solid line ) , interquartile range ( box ) , and range ( whiskers ) . the healthy control group demonstrated lower basal proinsulin - processing rates than the pancreas ( p < 0.01 ) and ii - isl ( p < 0.05 ) groups [ 74  14% vs. 83  7% vs. 86  12% , respectively , for insulin/(total proinsulin+insulin ) and 54  25% vs. 67 18% vs. 76  16% , respectively , for proinsulin fragments / total proinsulin ] . the ir - isl group displayed a basal insulin/(total proinsulin+insulin ) rate of 75  14% and a basal proinsulin fragments / total proinsulin rate of 54  25% . a : box plots of the proinsulin - processing ratios [ ( insulin / total proinsulin+insulin ) and ( proinsulin fragments / total proinsulin ) ] in healthy control group , ir - isl group , ii - isl group , and pancreas group before arginine injection ( basal values ) . b : box plots of the proinsulin - processing ratios [ ( insulin / total proinsulin+insulin ) and ( proinsulin fragments / total proinsulin ) ] in healthy control group , ir - isl group , ii - isl group , and pancreas group during peak stimulation after arginine injection ( stimulated values ) . data are expressed as median ( solid line ) , interquartile range ( box ) , and range ( whiskers ) . i / tp+i , insulin / total proinsulin+insulin ; pf / tp , proinsulin fragments / total proinsulin . 3b . interestingly , healthy control subjects , pancreas recipients , and ii - isl recipients all attain similarly elevated processing ratios after stimulation ( 93  2% vs. 93 3% vs. 93  5% , respectively , for insulin/(total proinsulin+insulin ) and 80  7% vs. 77 9% vs. 76  12% , respectively , for proinsulin fragments / total proinsulin ) , while ir - isl transplant recipients ratios do not demonstrate a comparable increase . although ir - isl proinsulin - processing ratios do increase slightly after stimulation to 87  4% and 58  18% for insulin/(total proinsulin + insulin ) and proinsulin fragments / total proinsulin , respectively , these values remain significantly lower than all other groups ( p < 0.01 vs. control and pancreas groups , p < 0.05 vs. ii - isl ) . this study explores for the first time proinsulin processing in islet transplant recipients , not only in comparison with healthy control subjects , but also with respect to pancreas transplant recipients . we have shown , first , that the exposure to arginine stimulation is in fact similar in ii - isl transplant recipients and healthy control subjects . as expected , recipients of pancreas transplants with systemic venous drainage demonstrate higher insulin response to arginine than healthy control subjects . second , there is a noticeable difference in the rate of basal proinsulin processing comparing ii - isl and ir - isl transplant recipients . ii - isl patients have proinsulin - processing ratios similar to those of pancreas transplant recipients , which are significantly higher than those of healthy control and ir - isl patients . third , after stimulation , ii - isl , pancreas , and healthy control subjects all increase their proinsulin - processing ratios to a similar level . in contrast , ir - isl patients are unable to achieve the same increase in proinsulin - processing rate . these data suggest that fully functional islet grafts are able to attain optimal proinsulin processing when stimulated but are required to sustain high levels of processing even during a metabolic fasting state in order to maintain glycemic control . islet grafts requiring supplementation with insulin therapy , on the other hand , demonstrate low basal proinsulin - processing rates , which are unable to reach optimal levels even when stimulated . interestingly , total proinsulin levels in this group ( ir - isl ) are low compared with other groups both before and after stimulation . however , the ratio of proinsulin to insulin in the ir - isl group after stimulation is disproportionately high . this suggests that partially functional islet grafts have a defect in proinsulin processing , which manifests itself as relative and not absolute hyperproinsulinemia . it should be noted that the calculation of proinsulin fragments may be slightly biased owing to elisa cross - reactivities . in theory , when we calculate proinsulin fragments as proinsulin fragments = total proinsulin  ipi , we are assuming that all of the des-64,65 proinsulin has been measured by the total proinsulin assay and that none of it has been measured by the intact assay . in reality , we measure only 81% of des-64,65 with the total proinsulin assay and 36% with the ipi assay . . however , since the pc2 pathway responsible for the formation of des-64,65 proinsulin is not the primary pathway for the production of insulin , unmeasured levels of des-64,65 proinsulin are relatively insignificant with respect to des-31,32 proinsulin levels . it should also be noted that clearance of proinsulin and proinsulin fragments could be altered by diminished renal function . as seen in table 1 , our ir - isl group demonstrated creatinine levels that were elevated compared with healthy control subjects . however , this difference was not significant comparing the ir - isl group to ii - isl and pancreas groups . in contrast , the reduced processing ratios observed in the ir - isl group are significantly lower than all other groups . both absolute and relative hyperproinsulinemia have already been observed in settings of impaired glucose metabolism ( 1214 ) . first , it has been suggested that dysfunctional -cells present a fundamental defect in the pathway responsible for processing of proinsulin to insulin ( 18 ) . it is possible in the case of islet transplantation that manipulation of islets , introduction of -cells into a foreign microenvironment , or immunosuppressive drugs may render these cells inherently dysfunctional . a second theory suggests that hyperproinsulinemia occurs because of increased demand on -cells , which leads to insufficient time to complete proinsulin processing intracellularly before granule secretion occurs ( 15,19,20 ) . it seems reasonable to suppose that a combination of insulin resistance and decreased -cell mass could lead to unattainable insulin needs and eventual -cell deterioration via exhaustion . our data show that islet grafts initially present increased basal proinsulin processing with respect to healthy control subjects and that this processing deteriorates as graft function declines , a phenomenon that more readily supports the latter theory . we can not , however , exclude the effect of prolonged glucotoxicity on -cell function as an additional possible explanation for the observed processing defects . although our two islet groups had similar fasting glucose levels , hba1c was slightly increased in the ir - isl group , indicating that chronic glucotoxicity may have played a role in the functional deterioration of -cells within this subgroup . ultimately , a longitudinal study on the same or similar populations might help to verify this hypothesis . two studies have previously investigated proinsulin processing in islet transplant recipients ( 16,22 ) . however , neither study used arginine stimulation to explore hormonal processing when under simulated metabolic stress and neither study compared islet and pancreas transplant recipients . it is interesting to note that these studies produced conflicting data and presented contrasting conclusions . we found that functional islet grafts display increased processing and relatively lower proinsulinemia at rest , with an inability to increase their processing rates further when stimulated . these grafts are essentially behaving the same during the metabolic fasting state as fully stimulated healthy control subjects . as islet grafts lose function , however , processing  even during the fasting state  becomes less effective and proinsulinemia increases . these findings directly correlate with the findings of mcdonald et al . and are supported by the data of klimek at al . , which include mostly ir - isl transplant recipients . two other groups , fiorina et al . it would seem that there is a threshold level of demand beyond which -cells can no longer process proinsulin effectively . as stated earlier , we presume this to be due to an increased demand on decreased -cell mass . further investigation is warranted to explain the exact mechanism responsible for the increased basal proinsulin processing seen in ii - isl grafts . again , a longitudinal study of these patients might help shed more light on the issue . no study previously analyzed the stimulated hormonal response of pancreas grafts with respect to islet grafts . our data show that pancreas grafts behave quite similarly to healthy control subjects but with elevated overall hormone levels . this is undoubtedly a reflection of the systemic venous pancreatic drainage of these patients , in whom there is no hepatic first - pass metabolism of pancreatic hormonal secretions ( 23 ) . pancreas grafts tend to show similar patterns of proinsulin processing to fully functional islet grafts . whether this is peculiar to systemically drained pancreata can not be elucidated from this data . this study used arginine stimulation rather than oral or intravenous glucose tolerance tests to test secretory reserve in our patient populations . arginine stimulation provides the advantage of not inducing hyperglycemia , thus reducing the confounding influence of glucotoxicity to a minimum . other investigators have found the intravenous glucose tolerance tests to be more robust and a better indicator of -cell status ( 24 ) , but this method does not account for host insulin resistance . in summary , ii - isl transplant recipients can maintain basal metabolic parameters similar to healthy control subjects at the cost of a higher rate of basal proinsulin processing . ir - isl transplant recipients demonstrate both lower insulin response and lower basal rates of proinsulin processing , which remain suboptimal even after arginine stimulation ( i.e. , loss of graft function is associated with less effective processing and relative hyperproinsulinemia ) . finally , the higher airs of pancreas transplant recipients are a reflection of systemic venous drainage of endocrine secretions .
objective-cells have demonstrated altered proinsulin processing after islet transplantation . 
 we compare -cell metabolic responses and proinsulin processing in pancreas and islet transplant recipients with respect to healthy control subjects.research design and methodswe studied 15 islet and 32 pancreas transplant recipients . 
 islet subjects were subdivided into insulin - requiring ( ir - isl , n = 6 ) and insulin - independent ( ii - isl , n = 9 ) groups . 
 ten healthy subjects served as control subjects . 
 subjects were administered an intravenous arginine stimulation test , and insulin , c - peptide , total proinsulin , intact proinsulin , and proinsulin fragment levels were determined from serum samples . acute insulin response ( air ) and proinsulin 
 processing rates were calculated.resultswe found that basal insulin and c - peptide levels were higher in the pancreas group than in all other groups . 
 ii - isl patients had basal insulin and c - peptide levels similar to healthy control subjects . 
 the ir - isl group had significantly lower airs than all other groups . 
 basal processing rates were higher in the pancreas and ii - isl groups than in healthy control subjects and the ir - isl group . 
 after arginine stimulation , all groups had elevated processing rates , with the exception of the ir - isl group.conclusionsour data suggest that ii - isl transplant recipients can maintain basal metabolic parameters similar to healthy control subjects at the cost of a higher rate of proinsulin processing . 
 ir - isl transplant recipients , on the other hand , demonstrate both lower insulin response and lower basal rates of proinsulin processing even after arginine stimulation .