Objective: Our purpose was to evaluate the role of calcium and calcium channels in endothelin-1-induced contraction of the smooth muscle of human placental veins.
Study design: Placentas were collected after vaginal delivery at term. After their removal from the chorionic plate, placental veins were divided into rings that were suspended in organ chambers and stretched to optimal tension. In the first part of the study, vessels from six women were initially suspended in calcium-poor modified Krebs-Ringer solution. They were then treated with either EGTA [ethylene glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid; calcium chelator, 0.5 mmol/L] or calcium chloride 2.5 mmol/L (control). Endothelin-1 was then added cumulatively (10(-10) to 10(-7) mol/L), and the resulting changes in isometric tensions were recorded. In the second part of the study vessels from six other women were treated with either (1) normal modified Krebs-Ringers solution (control), (2) calcium-poor modified Krebs-Ringers solution, or (3) nicardipine (dihydropyridine calcium channel inhibitor, 10(-7) mol/L) in separate organ chambers. Endothelin-1 was then added cumulatively.
Results: Endothelin-1 produced concentration-dependent contractions in placental veins, with maximal tension reached at 10(-7) mol/L. Substitution of calcium-poor for standard Krebs-Ringers solution in the organ chamber abolished contractions to low endothelin-1 concentrations (< or = 10(-9) mol/L, p < 0.001) but did not affect the contractile response to higher concentrations. EGTA abolished contractions to all concentrations tested (p < 0.02). Nicardipine significantly, but incompletely, inhibited the contractile responses to all endothelin-1 concentrations tested (p < 0.05).
Conclusions: Endothelin-1 induces contraction of the smooth muscle of human placental veins, which requires the influx of extracellular calcium. Dihydropyridine-sensitive calcium channels represent a major route of entry, but other pathways participate. The fetal effects of nifedipine and other calcium-channel blockers deserve specific evaluation in intrauterine growth retardation and other pregnancies complicated by elevated fetal levels of endothelin-1.