AGONIST-SPECIFIC BEHAVIOR OF THE INTRACELLULAR CA2+ RESPONSE IN RAT HEPATOCYTES

A variety of agonists stimulate in hepatocytes a response that takes t he shape of repetitive cytosolic free Ca2+ transients called Ca2+ osci llations. The shape of spikes and the pattern of oscillations in a giv en cell differ depending on the agonist of the phosphoinositide pathwa y that is applied. In this study, the response of individual rat hepat ocytes to maximal stimulation by arginine vasopressin (AVP), phenyleph rine and ADP was investigated by fluorescence microscopy and flash pho tolysis. Hepatocytes loaded with Ca2+-sensitive probes were stimulated with a first agonist to evoke a maximal response, and then a second a gonist was added. When phenylephrine or ADP was used as the first agon ist, AVP applied subsequently could elicit an additional response, whi ch did not happen when AVP was first applied and phenylephrine or ADP was applied later. Cells microinjected with caged myo-inositol 1,4,5-t risphosphate (IP3) were challenged with the different agonists and, wh en a maximal response was obtained, photorelease of IP3 was triggered. Cells maximally stimulated with AVP did not respond to IP3 photorelea se, whereas those stimulated with phenylephrine or ADP responded with a fast Ca2+ spike above the elevated steady-state level, which was fol lowed by an undershoot. In contrast, with all three agonists, IP3 phot orelease triggered at the top of an oscillatory Ca2+ transient was abl e to mobilize additional Ca2+ These experiments indicate that the diff erential response of cells to agonists is found not only during Ca2+ o scillations but also during maximal agonist stimulation and that poten cy and efficacy differences exist among agonists.