ALTERATIONS IN CYTOPLASMIC CALCIUM SENSITIVITY DURING PORCINE CORONARY-ARTERY CONTRACTIONS AS DETECTED BY AEQUORIN

1. Intracellular‐Ca2+‐force relationships were investigated in porcine epicardial coronary arteries by the simultaneous measurement of aequorin luminescence and isometric force. 2. In response to K+ depolarization and histamine, force and aequorin light rose monophasically. In response to carbachol and serotonin, tonic contractions were accompanied by biphasic aequorin signals consisting of an initial spike followed by a low plateau. Contractions produced by prostaglandin F2 alpha (PGF2 alpha) or the endoperoxide analogue U‐46619 were accompanied by little or no detectable rise in light. 3. Comparison of steady‐state force to steady‐state light levels indicated that agonists gave greater force for a given intracellular Ca2+ concentration ([Ca2+]i) compared to that seen during K+ contractures. 4. In Ca2+‐free bathing media, carbachol produced a transient contraction accompanied by a transient intracellular Ca2+ spike indicating release of Ca2+ from intracellular storage sites. 5. In Ca2+‐free bathing media PGF2 alpha produced a tonic contraction with no detectable change in light. 6. These results suggest that changes in the sensitivity of the contractile apparatus to Ca2+ or other activator systems may be as important a mechanism of contraction as are changes in [Ca2+]i. © 1987 The Physiological Society