CALCIUM DEPENDENCE OF EFFECTS OF ENDOTHELIN ON RAT MESENTERIC MICROVESSELS

We investigated the calcium dependence of the effects of endothelin (ET) on resistance vessels (less than 300-mu-m lumen diameter) from the mesenteric vascular bed of the rat, mounted on a wire myograph. ET-1 induced a potent sustained contraction with an ED50 of 12 nmol/L. The response to ET-3 and big ET at the maximum concentrations used (100 nmol/L) was less than 40% of that to ET-1, with an estimated ED50 of 45 nmol/L. Relaxation of the ET-1-induced contraction was slow, and resulted in a reduction of the maximum response to a second challenge with ET-1 to 60% of the initial contraction after 3 h. Long-lasting tachyphylaxis to arginine vasopressin (AVP) induced contraction also occurred. The response to 100 nmol/L ET-1 produced an active tension 88% greater than that induced by 124 mmol/L KCl, and similar to that produced by norepinephrine and AVP. The response to 100 nmol/L ET-1 in the absence of calcium + 1 mmol/L EGTA in the medium for 30 min resulted in a maximum contraction of 43% of the response in the presence of calcium, followed by a faster relaxation rate. The addition of calcium produced a further contraction, and stimulation with 100 nmol/L ET-1 at this point did not result in further response. The calcium channel blocker nitrendipine in concentrations of 1-10-mu-mol produced increasing reductions of the responses to 100 nmol/L ET-1 to 35% at the higher concentration. Nitrendipine (3-mu-mol/L) partially blocked the response to calcium after ET-1 was added in the absence of calcium. We conclude that resistance microvessels of the rat mesentery are more sensitive to ET-1 than ET-3 or big ET, which were also much less potent. These microvessels have a sensitivity to AVP 20 times greater than that to ET-1. The response to ET-1 may be mediated in part by the release of intracellular calcium, but the influx of calcium from the extracellular fluid through plasma membrane voltage-dependent channels is necessary for completing approximately 60% of the initial contraction and for the persistence of a sustained response.