INTRACELLULAR ATP AND GTP ARE BOTH REQUIRED TO PRESERVE MODULATION OFN-TYPE CALCIUM-CHANNEL CURRENT BY NOREPINEPHRINE

Norepinephrine (NE) inhibits voltage-dependent calcium channels of sym pathetic neurons. We investigated the role of intracellular nucleotide s in this inhibition for clues to receptor-channel coupling mechanisms . Both ATP and GTP are required to preserve NE responsiveness during w hole-cell dialysis. The response to NE was gradually lost in bullfrog sympathetic neurons dialyzed with GTP as the only nucleotide, ATP only , or no nucleotides. Replacing ATP with ATP[gamma-S] resulted in spont aneous modulation of calcium channel current, possibly because of prod uction of GTP[gamma-S]. The nonhydrolyzable ATP analog p[NH]ppA could substitute for ATP to preserve NE responsiveness. The protein phosphat ase inhibitors okadaic acid and calyculin-A did not affect NE inhibiti on of calcium channel current, or recovery from that inhibition. These results suggest protein phosphorylation is not involved in the inhibi tion of calcium channel current, but binding of ATP to some intracellu lar site is required for the coupling of adrenergic receptors to calci um channels.