Thimerosal enhances agonist-specific differences between [Ca2+]i oscillations induced by phenylephrine and ATP in single rat hepatocytes

Single rat hepatocytes, microinjected with the Ca2+-sensitive photoprotein aequorin, respond to agonists acting through the phosphoinositide signalling pathway by the generation of oscillations in cytosolic free Ca2+ concentration ([Ca2+](i)). The duration of [Ca2+](i) transients generated is characteristic of the stimulating agonist; the differences lie in the rate of fall of [Ca2+](i) from its peak. We considered that differential sensitivity of the InsP(3) receptor may underlie agonist specificity. The thiol reagent, thimerosal, is known to increase the sensitivity of the Ca2+ stores to InsP(3) by increasing the affinity of the InsP(3) receptor for InsP(3) in rat hepatocytes. We show here that a low dose of thimerosal (1 mu M), insufficient alone to elevate [Ca2+](i), potentiates [Ca2+](i) oscillations induced by phenylephrine or ATP in single, aequorin-injected, rat hepatocytes. Moreover, thimerosal enhances both the frequency and amplitude of phenylephrine-induced oscillations, whereas, in contrast, ATP-induced oscillations undergo an increase in the duration of the falling phase of individual [Ca2+](i) transients. Thimerosal, therefore, enhances, rather than eliminates, agonist-specific differences in the hepatocyte [Ca2+](i) oscillator.