SOME EVIDENCE AGAINST THE INVOLVEMENT OF ARACHIDONIC-ACID IN MUSCARINIC SUPPRESSION OF VOLTAGE-GATED CALCIUM-CHANNEL CURRENT IN GUINEA-PIG ILEAL SMOOTH-MUSCLE CELLS

1 To see if arachidonic acid (AA) plays a role in the sustained suppre ssion of voltage-gated calcium channel currents produced by muscarinic receptor stimulation by carbachol (CCh), the effects of AA on membran e currents were examined in whole-cell voltage-clamped smooth muscle c ells of the guinea-pig ileum. 2 In cells bathed in Ba2+ PSS and dialys ed with Cs+-based low EGTA (0.05 mM) pipette solution, and in which Ba 2+ current (I-Ba) flowing through voltage-gated calcium channels was e voked repeatedly by stepping to 0 mV from the holding potential of -60 mV, AA (1-30 mu M), applied extracellularly, gradually suppressed I-B a in a concentration-dependent manner. The I-Ba suppression was observ ed even with 20 mM EGTA in the pipette. 3 AA (3 mu M) and CCh (10 mu M ) shifted the voltage-dependent inactivation curve of I-Ba in the nega tive potential direction, but the effect of AA differed from that of C Ch in that an accompanying appreciable decrease in the slope was obser ved. 4 The sustained suppression of I-Ba induced by CCh (10 mu M) rema ined almost unaltered after pretreatment with 4-bromophenacyl bromide (10 mu M), an inhibitor of phospholipase AZ, or combination of indomet hacin (10 mu M), an inhibitor of the cyclo-oxygenase pathway, and nord ihydroguaiaretic acid (10 mu M), an inhibitor of the lipoxygenase path way. 5 In cells bathed in Ca2+ PSS and dialysed with K+-based pCa 6.5 pipette solution, voltage-dependent Ca2+ current (I-Ca) and K+ current (I-K) were recorded simultaneously. AA (3 mu M) suppressed I-K as wel l as I-Ca, whereas CCh (10 mu M)) suppressed I-Ca but not I-K 6 We con clude from these results that AA or its metabolite is unlikely to be i nvolved in the sustained suppression of voltage-gated calcium channel current induced by muscarinic receptor stimulation in guinea-pig ileal smooth muscle cells.