MODULATION OF DIHYDROPYRIDINE RECEPTORS IN VASCULAR SMOOTH-MUSCLE CELLS BY MEMBRANE-POTENTIAL AND CELL-PROLIFERATION

We have studied binding of isradipine to A7r5 vascular smooth muscle c ells as a function of membrane potential and cell proliferation. Consi stent with a voltage-modulated receptor model, two classes of binding sites were detected in confluent cultures: high-affinity sites under d epolarizing (50 mM K+) conditions (K-d = 45 +/- 3 pM), and lower affin ity sites under resting (5 mM K+) conditions (K-d = 181 +/- 20 pM). Ho wever, proliferating cells also displayed the high-affinity state at r est (K-d = 29 +/- 9 pM) in addition to a low-affinity site (K-d = 869 +/- 383 pM). Analysis of dissociation rates also revealed two receptor classes during proliferation. Proliferating cells showed a single cla ss of high-affinity sites (K-d = 39 +/- 6 pM) when depolarized, simila r to confluent cells. Receptor density in confluent monolayers increas ed from 15 +/- 3 fmol/10(6) cells at 5 days to 72 /-+ 6 fmol/10(6) cel ls after 10 days. These results suggest (i) that some L-type Ca2+ chan nels are spontaneously active in proliferating vascular smooth muscle cells, but require depolarization to activate in a confluent monolayer , and (ii) that the density of dihydropyridine receptors increases aft er a monolayer becomes confluent.