Three morphologically distinct types of muscle fibres isolated from the platyhelminth Schistosoma mansoni have been studied with whole cell current- and voltage-clamp techniques. Fibres showed a marked time-dependent decrease in membrane resistance in response to depolarizing current injections. Voltage-clamp experiments revealed the presence of two distinct voltage-gated outward currents. The most prominent current is a slowly activating, slowly and incompletely inactivating potassium current similar to delayed rectifier currents which have been described in a variety of cell types from a variety of organisms. Also present is a faster activating, quickly and completely inactivating potassium current that shares functional characteristics with ‘A’-currents. All three of the cell types studied possess a delayed rectifier current, but only two of the three types have ‘A’-currents. Though depolarization with high K+ leads to contraction of the dispersed fibres, no voltage-gated inward currents could be detected by whole cell voltage-clamp under any of our conditions.

Crayfish photoreceptors exhibit a voltage-dependent potassium conductance, GK, that is generally similar to the delayed rectifier channel described in neurons and other arthropod retinular cells. GK activation (i.e. the apparent threshold, Vth) occurs near the resting potential and GK is substantially reduced by 25 mM extracellular tetraethylammonium (TEA) and by intracellular Cs+ injections. Light exposure, sufficient to reduce visual sensitivity 100-fold, increases Vth (shifts it in the depolarizing direction) by about 20 mV. The light-dependent change in Vth does not depend upon the corresponding increase (depolarization) of the steady-state membrane potential nor does it depend upon inward calcium currents. Vth is slightly influenced by fluctuations in Ko associated with the light-elicited currents. During light exposure Ko (measured with K+-sensitive electrodes) increases by 2.1 mM (equivalent to an 8 mV increase in EK). This increase in EK makes only a modest contribution to the light-dependent change in Vth as determined by perfusion with high potassium salines. Intracellular calcium injections increase Vth by 10 to 20 mV and reduce visual sensitivity by 5- to 10-fold. The results imply that during exposure to high levels of illumination, K+ currents at the steady-state membrane potential are diminished by a calcium-dependent change in GK gating and, to a smaller degree, by a reduced K+ concentration gradient. It is notable that Ca2+ appears to inhibit both GK and the light-elicited conductance from both inside and outside the plasma membrane. As a consequence of the light-dependent change in Vth, GK makes only modest contributions to the changes in sensitivity and speed normally associated with light adaption. These functions are regulated by the transduction pathway and are revealed at the resting potential in the time course and magnitude of the light-elicited currents.

The electroretinogram (ERG) is generated by light-induced electrical activity in retinal cells. Since potassium ions and potassium conductances play a major role in determining the membrane potential of cells, changes in these are expected to affect the amplitude and pattern of the ERG. We recorded the ERG responses and the isolated P-III waves of rabbits after intraocular injections of specific blockers for potassium channels. 4-aminopyridine (4-AP) did not cause any noticeable changes in the ERG while tetraethylammonium chloride (TEA) induced time-dependent effects. Short-term (1–2 h) effects were expressed as significant augmentation of the b-wave with little change in the a-wave. At longer periods of follow-up, the a-wave increased in amplitude while the b-wave decreased. TEA augmented the amplitude of the isolated P-III wave. These effects of TEA can be explained by TEA-induced depolarization of the photoreceptors. Cesium ions and barium ions induced substantial augmentation of the b-wave. Barium but not cesium ions reduced the isolated P-III component of the ERG probably by blocking the potassium channels in the Müller cells. The augmentation of the b-wave by both barium or cesium ions is inconsistent with the Müller cells hypothesis for the ERG b-wave.