ATP AND G-PROTEINS AFFECT THE RUNUP OF THE CA2+ CURRENT IN BOVINE CHROMAFFIN CELLS

The Ca2+ current recorded by the whole-cell technique in chromaffin ce lls shows, before the often described rundown, a transient facilitatio n or runup. Initial current amplitude was 570 +/- 165 pA and then it i ncreased by 49 +/- 23% (n = 19. SD) over 2 +/- 1 min in the absence of adenosine 5'-triphosphate (ATP). In the presence of ATP, this process occurred with the same magnitude but it was slowed in a dose-dependen t manner, lasting 17 +/- 2 min with mM ATP (n = 8). Since adenosine 5' -diphosphate (ADP) does not reproduce this ATP effect, a complex serie s of phosphorylations is likely to intervene and we show that, at leas t, a cAMP-dependent i.e., cyclic adenosine monophosphate, phosphorylat ion occurs. Pertussis toxin (PTX) pretreatment yielded an already maxi mal Ca2+ current (around 1000 pA) at the time of the patch rupture, wh ich only slightly increased thereafter (10%, n = 11). Also, guanosine 5'-diphosphate (GDP) and guanosine 5'-O-(2-thiodiphosphate) (GDP[beta s]), induced a fast runup, which was absent in the presence of GTP. Fu rthermore, we show that facilitation does not occur in the presence of dihydrophyridine (DHP) antagonists. Globally, our data suggest that a n ATP-dependent phosphorylation stabilizes the inhibitory control exer ted by a PTX-sensitive G protein and, as a result, slows down the faci litation of l-type Ca2+ channels. The recruitment of L-type channels c an also be facilitated by the application of a DHP agonist or a depola rizing prepulse protcool. We show that these processes are only effect ive over a period which parallels the runup and are not additive to it . This suggests that a single process may underlie these various types of facilitation.