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

Functional responses and subcellular calcium redistribution were compared in extramural canine coronary arteries to determine the ultrastructural source of calcium for depolarization-induced contractions. The subcellular distribution of total (bound and free) Ca-45 in coronary artery smooth muscle was determined using Ca-45 electron microscopic autoradiography procedures described previously (Wheeler-Clark et al., 1986). Relative Ca-45 activities were compared for ultrastructural regions that included the plasma membrane (PM) region and sarcoplasmic reticulum in canine coronary smooth muscle frozen in control and high K+ solutions 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 cells; thus, sarcoplasmic reticulum Ca2+ release was not observed as a result of K+-induced contraction in canine coronary arteries. However, the Ca-45 activity of the PM was reduced by 75% in K+-contracted cells (P < .05). Inasmuch as nitrendipine completely inhibited both contraction 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 of K+-contracted cells is due to Ca2+ redistribution from the pericellular space into the cytosol during Ca2+ channel activation. Data obtained using electron probe x-ray microanalysis also indicate that extracellular Ca2+ loss was restricted to the pericellular space within 100 nm 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 enters the cell through activated, Ca2+ channels and 2) a diffusion barrier at the outer edge of the glycocalyx promotes and prolongs this calcium buffering.