Calcium Sensitivity and Agonist-Induced Calcium Sensitization in Small Arteries of Young and Adult Spontaneously Hypertensive Rats
Authors: Shaw Linda M. , Ohanian Jacqueline , Heagerty Anthony M. 
Published: June 2012

Abstract
The sensitivity of the myofilaments to Ca
2+
is increased during agonist-induced contraction of vascular smooth muscle. Given the important contribution of vascular tone to the elevation of peripheral resistance observed in genetic hypertension, we have investigated whether alterations in myofilament Ca
2+
sensitivity occur in small arteries from spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto (WKY) controls during the developmental and established phases of hypertension. Segments of mesenteric, renal, and femoral artery with an average lumen diameter <300 μm from 5- or 20-week-old rats were mounted in a wire myograph. Morphological measurements were made and the vessels permeabilized with
Staphylococcus aureus
α-toxin. Dose-response curves to increasing concentrations of Ca
2+
were obtained and the ability of 100 nmol/L endothelin-1 (ET-1) or 10 μmol/L norepinephrine (NE) in the presence of 10 μmol/L GTP to enhance tension in response to low Ca
2+
(pCa6.7) was determined. Systolic, diastolic, and mean blood pressures were higher in SHR than in WKY at 5 and 20 weeks. The media thickness:lumen diameter ratio was increased in mesenteric and femoral arteries from SHR compared with WKY at 5 and 20 weeks. There was no difference in media thickness:lumen diameter ratio in renal arteries or between 5- and 20-week animals in any vascular bed. The pCa curves were not different in mesenteric, renal, or femoral arteries from hypertensive compared with normotensive rats or between age groups, except in femoral arteries at 20 weeks, which exhibited a greater sensitivity to Ca
2+
in SHR. Tension developed in response to maximal Ca
2+
(pCa5.0) was greater in permeabilized mesenteric arteries from SHR compared with WKY at 20 weeks of age only; media stress was again similar in both strains but increased in older animals compared with younger animals in mesenteric arteries from WKY. The submaximal contraction induced by pCa6.7 was greater in femoral and renal than mesenteric arteries. GTP (10 μmol/L) augmented the tension developed to pCa6.7 in mesenteric arteries at 5 and 20 weeks and in renal arteries at 20 weeks. Addition of 100 nmol/L ET-1 or 10 μmol/L NE in the continued presence of GTP markedly increased tension in mesenteric arteries at 5 and 20 weeks. In renal arteries, 10 μmol/L NE enhanced Ca
2+
sensitivity in the presence of GTP in SHR at 5 and 20 weeks and WKY at 5 weeks. In femoral arteries, there was a tendency for ET-1 and NE to increase Ca
2+
sensitivity, but this increase was significant in WKY at 20 weeks (ET-1) and SHR at 5 weeks (NE) only. We have demonstrated that the sensitivity of the myofilaments to Ca
2+
and ET-1– or NE-induced Ca
2+
sensitization is not different in permeabilized small mesenteric, renal, or femoral arteries from SHR compared with WKY controls. Only in SHR mesenteric arteries at 20 weeks of age was there evidence of increased active tension in response to maximal Ca
2+
, despite structural differences, consistent with increased muscle mass in femoral arteries from SHR. We conclude that it is unlikely that a ubiquitous abnormality of the sensitivity of the contractile apparatus to Ca
2+
or agonist-induced Ca
2+
sensitization in vascular smooth muscle underlies the elevated total peripheral resistance associated with hypertension.