Calcium channel activation in arterioles from genetically hypertensive rats

Enhanced contractile responsiveness to the calcium channel agonist Bay K 8644 has been documented in large conduit arteries and small muscular arteries from hypertensive rats. The present study examined the effects of Bay K 8644 on the intracellular calcium concentration ([Ca²⁺]_i) in microvessels from stroke-prone spontaneously hypertensive rats and normotensive Wistar-Kyoto rats. Using microspectrofluorometry of fura-2, [Ca²⁺]_i was measured in smooth muscle cells localized on arteriolar fragments (15-35 μm external diameter) isolated after collagenase digestion of the pancreas. Resting [Ca²⁺]_i in hypertensive arterioles (94±6 nM, n=29) did not differ from that in normotensive vessels (81±4 nM, n=40). KCl (50 mM), applied alone and in the presence of Bay K 8644 (30 nM), stimulated increases in [Ca²⁺]_i that were reversed in calcium-free solution and with nifedipine (10 μM), consistent with activation of potential-operated calcium channels. Potassium-induced calcium transients were consistently potentiated by Bay K 8644. The change in [Ca²⁺]_i evoked by KCI alone or in combination with Bay K 8644 did not differ between arterioles from hypertensive and normotensive rats. In 24% of the vessels from hypertensive rats and in 29% of those from normotensive rats, Bay K 8644 evoked an increase in [Ca²⁺]_i that did not differ significantly between the two strains. The findings indicate that, in contrast to observations made in larger arteries, there is no evidence of a functional abnormality in potentialoperated calcium channels in very small arterioles from genetically hypertensive rats.