Voltage-operated calcium channels are essential for the myogenic responsiveness of cannulated rat mesenteric small arteries

The role of L-type voltage-operated Ca2+ channels (VOCs) in myogenic responsiveness was studied in cannulated rat mesenteric small arteries [mean diameter at 100 mm Hg and full dilation was 329 +/- 9 (SE) micrometer]. Twenty-six arteries were cannulated and pressurized. The luminal cross-sectional area of these vessels was monitored continuously. To test for myogenic responsiveness, pressure was raised stepwise from 20 to 60 and from 60 to 100 mm Hg. Pressure elevation enhanced the vascular tone, reflecting spontaneous myogenic responsiveness. Nifedipine (1 and 10 microM) suppressed spontaneous myogenic responses. The alpha1-adrenoceptor agonist phenylephrine (1 and 10 microM), when administered at 20 mm Hg, elicited constriction and vasomotion, and potentiated myogenic constriction to subsequent pressure elevation. Nifedipine (1 and 10 microM) also suppressed phenylephrine-potentiated myogenic responsiveness. Stimulation of VOCs with BAY K 8644 (10-300 nM) had no effect at 20 mm Hg, but augmented myogenic responsiveness. K+ (16-46 mM) caused concentration-dependent constrictions when administered at 20 mm Hg, and potentiated myogenic responsiveness when the pressure was raised from 20 to 60 mm Hg. Thus, any intervention that blocked the VOCs also blocked myogenic responses. Therefore, we conclude that VOCs are essential for the myogenic responsiveness of cannulated rat mesenteric small arteries