Calcium dependence of effects of endothelin on rat mesenteric microvessels

Abstract

We investigated the calcium dependence of the effects of endothelin (ET) on resistance vessels (less than 300 μm 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 μmol produced increasing reductions of the responses to 100 nmol/L ET-1 to 35% at the higher concentration. Nitrendipine (3 μmol/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.Key words: mesenteric arterioles, vasopressin, norepinephrine, nitrendipine, calcium channel blockers.