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 (I-sc) reflects, across the anterior aspect, the presence of anterior ly facing K+ conductance(s) plus the Na+-K+ pump current. Across the p osterior surface the I-sc is primarily carried by the movement of Nafrom the posteriprimarily carried by the movement of Na+ from the post erior bathing solution to the lens. Addition of acetylcholine (ACh) to the posterior hemichamber did not affect the translens electrical par ameters; but, its introduction to the anterior bath at 1 mu M immediat ely reduced the I-sc from 8.91+/-1.47 to 5.8+/-1.28 mu A cm(-2) and in creased the translens resistance from 1.50+/-0.08 to 1.59+/-0.09 K Ome ga cm(2) (+/- S.E.S; P < 0.05 as paired values, n = 25 lenses). The su ppressed I-sc gradually recovered and reached 75% of the control value 5 min after the introduction of the neurotransmitter. In six cases th e recovery was nearly complete (greater than or equal to 95% of contro l) within this time. The preaddition of 0.1 mu M atropine prevented an effect by 1 mu M ACh. When atropine was added within 1 min of ACh, th e suppressed I-sc immediately recovered. The ACh-elicited I-sc suppres sion was averted in lenses pre-exposed to either K+ channel blockers ( quinidine or barium) or to the endoplasmic reticular Ca2+-ATPase inhib itor thapsigargin (Tg; 0.1 mu M), which in itself produced I-sc inhibi tions similar to those seen with ACh under control conditions. Similar ly comparable were the ACh-evoked I-sc inhibitions garnered upon intro duction of the agonist to lenses bathed in the absence of extracellula r Ca2+. In these cases, however, the I-sc recovered fully within 2-3 m in. This condition also revealed that the anterior removal of medium C a2+ increased the I-sc by about 50%, a completely reversible phenomeno n; Ca2+ restoration in the presence of the Ca2+ channel blocker, nifed ipine (10 mu M), blunted markedly the reversal to the control I-sc. Ov erall, these results suggest that ACh receptor activation induces the release of intracellularly stored Ca2+, which in turn leads to the tem porary deactivation of a K+ conductance(s); in addition, secondary Ca2 + inflow may further extend the observed inhibition. During this study , the I(sc)s 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 mu A cm(-2); +/-S.D.S, n = 24). Experiments attempted to determine the sensitivity of the os cillatory activity to ACh, Tg, nifedipine, and the phorbol ester PMA. The latter two clearly inhibited the oscillations; ACh and Tg caused t emporary modifications, not overt inhibitions. It is suggested that AC h-induced cytoplasmic Ca+ fluctuations (described elsewhere) are not b y necessity linked to the translens current oscillations, although the latter apparently involves intermittent Ca2+ flows via a nifedipine-s ensitive pathway. The underlying nature and roles for these phenomena remain to be determined. (C) 1995 Academic Press Limited