L-TYPE AND CA2-DEPENDENT HIERARCHICAL CA2+ SIGNALING IN RAT PORTAL-VEIN MYOCYTES( RELEASE CHANNEL)

Ca2+ signalling events and whole-cell Ca2+ currents were analyzed in s ingle myocytes from rat portal vein by using a laser scanning confocal microscope combined with the patch-clamp technique. In myocytes in wh ich the intracellular Ca2+ store was depleted or Ca2+ release channels were blocked by 10 mu M ryanodine, inward Ca2+ currents induced slow and sustained elevations of [Ca2+](i). These Ca2+ responses were suppr essed by 1 mu M oxodipine and by depolarizations to +120 mV, a potenti al close to the reversal potential for Ca2+ ions, suggesting that they reflected Ca2+ influx through L-type Ca2+ channels. With functioning intracellular Ca2+ stores, flash photolysis of caged Ca2+ gave rise to a small increase in [Ca2+](i) with superimposed Ca2+ sparks, reflecti ng the opening of clustered Ca2+ release channels. Brief Ca2+ currents in the voltage range from -30 to +10 mV triggered Ca2+ sparks or macr osparks that did not propagate in the entire line-scan image. Increasi ng the duration of Ca2+ current for 100 ms or more allowed the trigger of propagating Ca2+ waves which originated from the same initiation s ites as the caffeine-activated response. Both Ca2+ sparks and initiati on sites of Ca2+ waves activated by Ca2+ currents were observed in the vicinity of areas that excluded the Ca2+ probes, reflecting infolding s of the plasma membrane close to the sarcoplasmic reticulum, as revea led by fluorescent markers. The hierarchy of Ca2+ signalling events, f rom submicroscopic fundamental events to elementary events (sparks) an d propagated waves, provides an integrated mechanism to regulate vascu lar tone.