Effects on the hepatocyte [Ca 2+] i oscillator of inhibition of the plasma membrane Ca 2+ pump by carboxyeosin or glucagon-(19–29)

Abstract

Single rat hepatocytes, microinjected with the Ca 2+-sensitive photoprotein aequorin, respond to agonists acting through the phosphoinositide signalling pathway by the generation of oscillations in cytosolic free Ca 2+ concentration ([Ca 2+] i). The duration of [Ca 2+] i transients generated is characteristic of the receptor species activated; the variability results in differences in the rate of fall of [Ca 2+] i from its peak. It is conceivable that the plasma membrane Ca 2+-ATPase (PIVI Ca 2+ pump) may have an important role in the mechanism underlying agonist specificity. It has recently been shown that an esterified form of carboxyeosin, an inhibitor of the red cell PIVI Ca 2+ pump, is suitable for use in whole cell studies. Glucagon-(19–29) (mini-glucagon) inhibits the Ca 2+ pump in liver plasma membranes, mediated by G s. We show here that carboxyeosin and mini-glucagon inhibit Ca 2+ efflux from populations of intact rat hepatocytes. We show that carboxyeosin and mini-glucagon enhance the frequency of oscillations induced by Ca 2+ mobilizing agonists in single hepatocytes, but do not affect the duration of individual transients. Furthermore, we demonstrate that inhibition of the hepatocyte PIVI Ca 2+ pump enables the continued generation of [Ca 2+] i oscillations for a prolonged period following the removal of extracellular Ca 2+.