CALCIUM-CHANNEL ACTIVATION MOBILIZES CALCIUM FROM A RESTRICTED PERICELLULAR REGION SURROUNDING CANINE CORONARY-ARTERY SMOOTH-MUSCLE CELLS

Functional responses and subcellular calcium redistribution were compa red in extramural canine coronary arteries to determine the ultrastruc tural source of calcium for depolarization-induced contractions. The s ubcellular distribution of total (bound and free) Ca-45 in coronary ar tery smooth muscle was determined using Ca-45 electron microscopic aut oradiography procedures described previously (Wheeler-Clark et al., 19 86). Relative Ca-45 activities were compared for ultrastructural regio ns that included the plasma membrane (PM) region and sarcoplasmic reti culum in canine coronary smooth muscle frozen in control and high K+ s olutions in the presence and absence of 3 x 10(-7) M nitrendipine. The Ca-45 activity of SR was similar in contracted and relaxed muscle cel ls; thus, sarcoplasmic reticulum Ca2+ release was not observed as a re sult of K+-induced contraction in canine coronary arteries. However, t he Ca-45 activity of the PM was reduced by 75% in K+-contracted cells (P < .05). Inasmuch as nitrendipine completely inhibited both contract ion and Ca-45 loss from the PM region of high Kf-depolarized cells, we suggest that the decreased relative Ca-45 activity in the PM region o f K+-contracted cells is due to Ca2+ redistribution from the pericellu lar space into the cytosol during Ca2+ channel activation. Data obtain ed using electron probe x-ray microanalysis also indicate that extrace llular Ca2+ loss was restricted to the pericellular space within 100 n m of the membrane bilayer. As a model to explain our data, we suggest that: 1) calcium bound to the glycocalyx buffers the free Ca2+ that en ters the cell through activated, Ca2+ channels and 2) a diffusion barr ier at the outer edge of the glycocalyx promotes and prolongs this cal cium buffering.