Release and recycling of calcium by the sarcoplasmic reticulum in guinea‐pig portal vein smooth muscle.

Abstract

The amplitude of interrupted contractions evoked by noradrenaline (norepinephrine) or caffeine in Ca2+-free, high-K+ solutions containing EGTA [ethyleneglycol-bis-(.beta.-aminoethylether)-N-N-tetraacetic acid] or La3+ was determined in small (40-60 .mu.m thick) bundles of guinea-pig portal anterior mesenteric vein. Interrupted contractions were produced by removing the stimulating agent as soon as the amplitude of the tension record reached its peak. The distribution of intracellular Ca2+ was determined, with electron probe X-ray microanalysis, in cryosections of preparations frozen in the relaxed state and at the peak of noradrenaline-induced contractions. Interrupted contractions of maximal or near-maximal amplitudes could be evoked every 2 min for up to 15 min in the virtual absence of extracellular Ca2+. If noradrenaline was allowed to remain in the solution throughout the period of spontaneous relaxation, a subsequent contraction could no longer be evoked in the absence of extracellular Ca2+. Interrupted contractions, similar to those evoked by noradrenaline, could also be stimulated by caffeine. In freeze-substituted smooth muscles, in which the membranes of the junctional sarcoplasmic reticulum could be visualized, the regions containing high Ca were identified as part of the sarcoplasmic reticulum (SR), indicating that the SR is the store from which noradrenaline and caffeine release Ca2+.