Effect of Ca2+ agonists in the perfused liver: determination via laser scanning confocal microscopy

Ca2+ is a critical intracellular second messenger, but few studies have exa mined Ca2+ signaling in whole organs. The amplitude and frequency of Ca2+ o scillations encode important cellular information. Using laser scanning con focal microscopy in the indo 1 acetoxymethyl ester dye-loaded rat liver, we investigated the effect of various Ca2+ agonists that act at distinct mech anistic sites on Ca2+ signaling. Perfusion with suprathreshold doses of arg inine vasopressin (AVP) (2-20 nM) caused a single Ca2+ wave that originated in the pericentral vein region and spread centrifugally to the periportal area. Lower doses of AVP (0.2-2 nM) caused multiple Ca2+ waves and Ca2+ osc illations. Perfusion with ATP (1.4-17.5 mu M) caused rapid transient elevat ions in intracellular free Ca2+ concentration ([Ca2+](i)) occurring in isol ated hepatocytes or groups of hepatocytes throughout the lobule and were of shorter duration than those due to AVP. Also in contrast to AVP, there was no specific anatomic location within the hepatic lobule that was more susc eptible to ATP Thapsigargin and cyclopiazonic acid did not cause a Ca2+ nav e but rather produced a uniform and fairly simultaneous increase in [Ca2+]( i) in all hepatocytes in the lobule. Perfusion with 14 mu M ryanodine produ ced a single transient spike in [Ca2+](i) in a small number (<2%) of hepato cytes. Dantrolene, an inhibitor of Ca2+ release, reduced the increased [Ca2 +](i) occurring after AVP. Insight into the mechanism of action of these Ca 2+-active compounds on Ca2+ signaling in the intact liver is provided.