Effect of the Ca(2+)-channel agonist Bay K 8644 on the contractile responses in human placental veins.

Abstract

1. The aim of the present study was to analyse, in segments of human placental veins, the effect of the Ca2+ channel agonist Bay K 8644 (0.1 microM) and Ca2+ channel antagonists nifedipine (0.1 microM) and diltiazem (1 microM) on vascular contractility and 45Ca2+ uptake. 2. The Ca2+ channel agonist Bay K 8644 (0.1 microM) caused small concentration dependent contractions that were increased by a moderate membrane depolarization with 7.5 mM K+. This increase was reversed by nifedipine and diltiazem. Ca2+ addition to segments previously depolarized with 75 mM K+ and exposed to a Ca(2+)-free medium caused contractile responses that were increased by 0.1 microM Bay K 8644; such an increase was blocked by 0.1 microM nifedipine and 1 microM diltiazem. 3. K+ and 5-HT induced concentration dependent contractile responses which were increased by Bay K 8644 (0.1 microM). Both 0.1 microM nifedipine and 1 microM diltiazem inhibited the increasing effect of Bay K 8644. Bay K 8644 (30 nM and 0.1 microM) caused an enhancement in 45Ca2+ accumulation over the basal value, that was increased by membrane depolarization with K+ (7.5, 15 and 30 nM) and inhibited by nifedipine (0.1 microM). K+ (15 and 30, but not 7.5 mM) and 5-HT (1 microM) induced 45Ca2+ uptake over the basal level that was increased by Bay K 8644 (0.1 microM). Such an increase was antagonized by nifedipine (0.1 microM). 4. These data indicate that: (1) a small depolarization with K+ is needed for Bay K 8644 to be able to produce consistent contractile responses, suggesting that voltage gated Ca2+ channels (VGCCs) are not activated in a basal situation in placental veins; (2) the increase of 5-HT contraction by Bay K 8644 may be produced by either the capability of this amine to depolarize the membrane of smooth muscle cells and subsequent facilitation of Ca2+ influx through VGCCs or direct activation by Bay K 8644 of receptor (5-HT) operated Ca2+ channels (ROCs), and (3) the increasing effect of Bay K 8644 appears to be due to a Ca2+ entry activation through VGCCs.