Repetitive calcium transients in hormone-stimulated cells

Abstract

Many hormones that act via the phosphoinositol .cycle induce transient rises in cytoplasmic free Ca2+ in a variety of cell types (Berridge et al., 1988). To this list may now be added vascular smooth muscle cells from rat and rabbit aorta in primary culture; both angiotensin I 1 and ATP induce repetitive free Ca2+ transients in spread single cells microinjected with aequorin (Fig. 1). The mechanisms by which cells generate oscillations in free Ca2 + have tentatively been classified (Berridge et ul., 1988) into either oscillations in which free Ca2+ is continuously part of the mechanism (e.g. Ca2+-induced release of Ca'+) or oscillations in the production of inositol 1,4,5-trisphosphate [Ins( 1,4,5)P,] in which free Ca2+ is not a continuous contributor to the oscillator. Our aequorin measurements of repetitive free CaL+ transients in single rat hepatocytes exposed to various hormones (Woods et ml., 1986) are most readily explained by transient receptor-mediated production of lns(1,4,5)lJ, (Woods et al., 1 9 8 7 ~ ; Cobbold et ul., 1987, 1988~1, b). We base this supposition on the key observation that different agonists induce free Ca2+ transients with subtly different timecourses, even in the same individual cell. Thus transients induced by phenylephrine, an a , -adrenergic agonist, are about 7 s long, while angiotensin 11 and [ArgxJvasopressin transients are about 1216 s long (Woods et al., 1 9 8 7 ~ ) . Interestingly, ATP and ADP induce transients of markedly different timecourse (approx. 40 s and 7 s, respectively) (Cobbold et ul., 1988~) . Thus the transients show receptor-specific timecourses, and possibly even agonist-specificity (if ATP and ADP can be shown to act on the same receptor). Transients induced by all the agonists have very similar rates of rise of free Ca'+ and similar peak free Ca2+, and neither parameter is dependent upon agonist concentration. The receptor-specific differences in transient duration reside in the rate of fall of free Ca2+ from its peak level back to resting (Woods et al., 1 9 8 7 ~ ; Cobbold et al., 1 9 8 8 ~ ) . How such receptor-specific information may be transmitted through the phosphoinositide cycle to mobilize intracellular Ca2+ is not known. We could, perhaps, propose that different receptors act on distinct populations of phosphoinositidase C (PIC) with different switch-off kinetics, or different substrate specificities, so as to generate different kinetics of curtailment of Ins( 1,4,5)lJ,-mediated Ca2+ mobilization. However, the very similar rates of rise of free Ca2+, and of peak free Ca2+, in each transient would suggest that different PIC populations are unlikely to be involved. Rather, a plausible source of negative feedback for curtailing Ins( I ,4,5)P, production lies in phosphorylation of receptors and/or their G proteins by protein kinase C (PKC). Differences in the kinetics of PKC phosphorylation of the different receptors or G proteins could then explain the different rates of fall of free Ca2+, assuming the instantaneous level of