HORMONE-REGULATED CA2-CELL PATCH-CLAMP( CHANNEL IN RAT HEPATOCYTES REVEALED BY WHOLE)

An inward current responsible for hormone regulated Ca2+ entry has bee n identified in cultured rat hepatocytes using whole cell patch clamp, Addition of 20 nM vasopressin or of 100 mu M ATP induced the inward c urrent, which could be observed more clearly after blocking an outward K+ current, This large outward K+ current, which appeared after addit ion of vasopressin or ATP, could be blocked either by replacing K+ wit h Cs+ in the external medium and in the pipette solution, or by simply including 0.5 mu M apamin in the K+-containing external medium. The o utward current appears to be carried by a Ca2+ activated K+ channel. I n the presence of apamin, hepatocytes pretreated with vasopressin in a Ca2+-free media reveal an inward current on addition of external Ca2 (5 mM). The current could also be elicited by addition of vasopressin when cells are preincubated in the presence of 5 mM external Ca2+. No current is seen on addition of Ca2+ in the absence of vasopressin. In itially, the inward current was ca 200-300 pA at -60 mV, but it declin ed rapidly over 3 min to ca 20 pA. The current approached zero, as an asymptote at positive potential, and appeared to be somewhat inwardly rectifying. Additions of 5 mM Mn2+ or 5 mM Ba2+ in place of Ca2+ produ ced little or no current. An inhibitor of ER Ca2+-ATPase, thapsigargin , could also trigger the cascade of events leadinq to plasma membrane conductance of Ca2+. The data suggest that hormone-stimulated Ca2+ ent ry into hepatocytes is mediated by a Ca2+-release activated channel hi ghly specific for Ca2+. This is the first demonstration of such a chan nel in hepatocytes, though similar ones have been described in mast ce lls, in vascular endothelial cells and T-lymphocytes.