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) 45Ca in coronary artery smooth muscle was determined using 45Ca electron microscopic autoradiography procedures described previously (Wheeler-Clark et al., 1986). Relative 45Ca 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 45Ca 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 45Ca activity of the PM was reduced by 75% in K(+)-contracted cells (P < .05). Inasmuch as nitrendipine completely inhibited both contraction and 45Ca loss from the PM region of high K(+)-depolarized cells, we suggest that the decreased relative 45Ca 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.