Acetylcholine modulation of the short-circuit current across the rabbit lens

Rabbit lenses were bathed within a bicameral Ussing-type chamber under short-circuit conditions. In this situation the short-circuit current (Isc) reflects, across the anterior aspect, the presence of anteriorly facing K+ conductance(s) plus the Na(+)-K+ pump current. Across the posterior surface the Isc is primarily carried by the movement of Na+ from the posterior bathing solution to the lens. Addition of acetylcholine (ACh) to the posterior hemichamber did not affect the translens electrical parameters; but, its introduction to the anterior bath at 1 microM immediately reduced the Isc from 8.91 +/- 1.47 to 5.84 +/- 1.28 microA cm-2 and increased the translens resistance from 1.50 +/- 0.08 to 1.59 +/- 0.09 K omega cm2 (+/- S.E.S; P < 0.05 as paired values, n = 25 lenses). The suppressed Isc gradually recovered and reached 75% of the control value 5 min after the introduction of the neurotransmitter. In six cases the recovery was nearly complete (> or = 95% of control) within this time. The preaddition of 0.1 microM atropine prevented an effect by 1 microM ACh. When atropine was added within 1 min of ACh, the suppressed Isc immediately recovered. The ACh-elicited Isc suppression was averted in lenses pre-exposed to either K+ channel blockers (quinidine or barium) or to the endoplasmic reticular Ca(2+)-ATPase inhibitor thapsigargin (Tg: 0.1 microM), which in itself produced Isc inhibitions similar to those seen with ACh under control conditions. Similarly comparable were the ACh-evoked Isc inhibitions garnered upon introduction of the agonist to lenses bathed in the absence of extracellular Ca2+. In these cases, however, the Isc recovered fully within 2-3 min. This condition also revealed that the anterior removal of medium Ca2+ increased the Isc by about 50%, a completely reversible phenomenon; Ca2+ restoration in the presence of the Ca2+ channel blocker, nifedipine (10 microM), blunted markedly the reversal to the control Isc. Overall, these results suggest that ACh receptor activation induces the release of intracellularly stored Ca2+, which in turn leads to the temporary deactivation of a K+ conductance(s); in addition, secondary Ca2+ inflow may further extend the observed inhibition. During this study, the Isc of about 30% of the lenses used spontaneously oscillated (common duration of 30 min, with a mean peak frequency of 0.76 +/- 0.32 cycle min-1 and mean amplitude of 4.07 +/- 2.65 microA cm-2; +/- S.D.S, n = 24). Experiments attempted to determine the sensitivity of the oscillatory activity to ACh. Tg, nifedipine, and the phorbol ester PMA.(ABSTRACT TRUNCATED AT 400 WORDS)