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 anteriorly facing K⁺ conductance(s) plus the Na⁺-K⁺ pump current. Across the posterior surface the I_sc 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 μm immediately reduced the I_sc from 8·91 ± 1·47 to 5·84 ± 1·28 μA cm^-2 and increased the translens resistance from 1·50 ± 0·08 to 1·59 ± 0·09 KΩ cm² (±s.e.s.; P < 0·05 as paired values, n = 25 lenses). The suppressed I_sc 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 (≥ 95% of control) within this time. The preaddition of 0·1 μm atropine prevented an effect by 1 μm ACh. When atropine was added within 1 min of ACh, the suppressed I_sc immediately recovered. The ACh-elicited I_sc suppression was averted in lenses pre-exposed to either K⁺ channel blockers (quinidine or barium) or to the endoplasmic reticular Ca²⁺-ATPase inhibitor thapsigargin (Tg; 0·1 μm), which in itself produced I_sc inhibitions similar to those seen with ACh under control conditions. Similarly comparable were the ACh-evoked I_sc inhibitions garnered upon introduction of the agonist to lenses bathed in the absence of extracellular Ca²⁺. In these cases, however, the I_sc recovered fully within 2-3 min. This condition also revealed that the anterior removal of medium Ca²⁺ increased the I_sc by about 50%, a completely reversible phenomenon; Ca²⁺ restoration in the presence of the Ca²⁺ channel blocker, nifedipine (10 μm), blunted markedly the reversal to the control I_sc. Overall, these results suggest that ACh receptor activation induces the release of intracellular 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 I_scs 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 μA 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. The latter two clearly inhibited the oscillations; ACh and Tg caused temporary modifications, not overt inhibitions. It is suggested that ACh-induced cytoplasmic Ca⁺ fluctuations (described elsewhere) are not by necessity linked to the translens current oscillations, although the latter apparently involves intermittent Ca²⁺ flows via a nifedipine-sensitive pathway. The underlying nature and roles for these phenomena remain to be determined.