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 suppression of voltage-gated calcium channel currents produced by muscarinic receptor stimulation by carbachol (CCh), the effects of AA on membrane currents were examined in whole-cell voltage-clamped smooth muscle cells of the guinea-pig ileum. 2. In cells bathed in Ba2+ PSS and dialysed with Cs(+)-based low EGTA (0.05 mM) pipette solution, and in which Ba2+ current (IBa) flowing through voltage-gated calcium channels was evoked repeatedly by stepping to 0 mV from the holding potential of -60 mV, AA (1-30 microM), applied extracellularly, gradually suppressed IBa in a concentration-dependent manner. The IBa suppression was observed even with 20 mM EGTA in the pipette. 3. AA (3 microM) and CCh (10 microM) shifted the voltage-dependent inactivation curve of IBa in the negative potential direction, but the effect of AA differed from that of CCh in that an accompanying appreciable decrease in the slope was observed. 4. The sustained suppression of IBa induced by CCh (10 microM) remained almost unaltered after pretreatment with 4-bromophenacyl bromide (10 microM), an inhibitor of phospholipase A2, or a combination of indomethacin (10 microM), an inhibitor of the cyclo-oxygenase pathway, and nordihydroguaiaretic acid (10 microM), an inhibitor of the lipoxygenase pathway. 5. In cells bathed in Ca2+ PSS and dialysed with K(+)-based pCa 6.5 pipette solution, voltage-dependent Ca2+ current (ICa) and K+ current (IK) were recorded simultaneously. AA (3 microM) suppressed IK as well as ICa, whereas CCh (10 microM) suppressed ICa but not IK. 6. We conclude from these results that AA or its metabolite is unlikely to be involved in the sustained suppression of voltage-gated calcium channel current induced by muscarinic receptor stimulation in guinea-pig ileal smooth muscle cells.