Probing the impact of isoflurane on acute pancreatic function with 52Mn-PET

167 Objectives Volatile anesthetics such as isoflurane have been shown to protect the myocardium against reversible and irreversible ischemic injury during interventional cardiac procedures. Unfortunately isoflurane has also been known to impair insulin release and glucose metabolism since the mid-1970s. The mechanism for suppression of insulin secretion is debated, but one prominent theory is that isoflurane directly prevents inhibition of adenosine triphosphate-sensitive potassium channels (KATP) preventing β-cell membrane depolarization. Without plasma membrane depolarization, voltage dependent calcium channels (VDCCs) do not open which prevents activation of insulin exocytosis. 52gMn (t1/2=5.59d, β+=29.4%) has been shown to behave chemically like Ca2+ allowing for rapid VDCC transport, making it well suited for β-cell function investigation. The objective of this work was to demonstrate the suppressive effect that isoflurane has on mouse pancreatic uptake of 52Mn, confirming the prevention of β-cell membrane depolarization. Methods 52Mn was produced by 16 MeV proton irradiation of natCr on a GE PETtrace. Targets were dissolved in concentrated HCl, and then 52Mn was isolated by ethanolic anion exchange chromatography. The final 52Mn product was eluted in 0.01 M pH 6.0 NaOAc. 1.0-1.5 MBq of 52Mn was used for each subject. Fasted ICR mice were divided into three treatment groups. Group 1 received an intravenous (IV) bolus of 52Mn. Group 2 received an intraperitoneal (IP) injection of glucose (1 mg/kg), then an IV bolus of 52Mn after 10 minutes had elapsed. Group 3 received an IV bolus of glucose (1mg/kg), then an IV infusion of 52Mn from 5-15 min post-glucose. Half of each group received treatment while awake (n=3-6), and half received treatment under isoflurane anesthesia (n=3, 2% isoflurane, 1 l/min). After a total of one hour had elapsed a static PET scan was collected, subjects were sacrificed, and ex vivo52Mn biodistribution was measured by gamma counting. On a subset of subjects, dynamic PET data was acquired during 52Mn IV bolus injection for kinetic analysis. PET regions of interest (ROIs) were manually placed over organs with significant 52Mn uptake. Results Pancreatic uptake of 52Mn was found to be significantly lower in all three groups under the influence of isoflurane anesthesia compared to non-anesthetized controls. In mice injected with only an IV bolus of 52Mn, pancreatic uptake decreased from 20.4±3.23 %ID/g to 7.3±1.7 %ID/g (p=0.0002) under isoflurane. In mice injected with IP glucose 10 minutes prior to a bolus injection of 52Mn, uptake decreased from 20.0±4.35 %ID/g to 13.2±2.04 %ID/g (p=0.028). In the third group, with mice injected with IV glucose 5 minutes prior to a 52Mn IV infusion lasting 10 minutes pancreatic uptake was found to be reduced from 24.0±2.5 %ID/g to 13.5±2.0 %ID/g (p=0.001) while under isoflurane anesthesia. PET ROI quantification was found to be in good agreement with ex vivo biodistribution results. From kinetic analysis of the left ventricle ROI on dynamic PET data, it was found that 52Mn cleared from the blood rapidly, with a clearance half-life of less than 20 seconds. Conclusions Significant suppression of pancreatic 52Mn uptake by isoflurane was observed for all three groups. This result supports the theory that isoflurane prevents β-cell plasma membrane depolarization by activating ATP-sensitive potassium channels. To our knowledge, this work constitutes the first direct VDCC-based evidence for this theory.