Modulation of a sustained calcium current by intracellular pH in horizontal cells of fish retina.

Abstract

A B S T R AC T A sustained high voltage-activated (HVA), nifedipine-and cadmiumsensitive calcium current and a sustained calcium action potential (AP) were recorded from horizontal cells isolated from catfish retina, pH indicator dyes showed that superfusion with NH4CI alkalinized these cells and that washout of NH4C1 or superfusion with Na-acetate acidified them. HVA current was slightly enhanced during superfusion of NH4C1 but was suppressed upon NH4C1 washout or application of Na-acetate. When 25 mM HEPES was added to the patch pipette to increase intracellular pH buffering, the effects of NH4CI and Na-acetate on HVA current were reduced. These results indicated that intracellular acidification reduces HVA calcium current and alkalinization increases it. Sustained APs, recorded with high resistance, small diameter microelectrodes, were blocked by cobalt and cadmium and their magnitude varied with extraceUular calcium concentration. These results provide confirmatory evidence that the HVA current is a major component of the AP and indicate that the AP can be used as a measure of how the HVA current can be modified in intact, undialyzed cells. The duration of APs was increased by superfusion with NH4C1 and reduced by washout of NH4C1 or superfusion with Na-acetate. The Na-acetate and NH4CI washout-dependent shortening of the APs was observed in the presence of intracellular BAPTA, a calcium chelator, IBMX, a phosphodiesterase inhibitor, and in Na-free or TEA-enriched saline. These findings provide supportive evidence that intracellular acidification may directly suppress the HVA calcium current in intact cells. Intracellular pH changes would thereby be expected to modulate not only the resting membrane potential of these cells in darkness, but calcium-dependent release of neurotransmitter from these cells as well. Furthermore, this acidification-dependent suppression of calcium current could serve a protective role by reducing calcium entry during retinal ischemia, which is usually thought to be accompanied by intracellular acidosis.

The weak bases NH3 and trimethylamine inhibit the medium and slow afterhyperpolarizations in rat CA1 pyramidal neurons

“…In contrast, the effects of NH 3 and TMeA to increase the magnitudes of depolarization-evoked Ca 2+ potentials, [Ca 2+ ] i transients and I Ca were attenuated by increasing b T . Although these findings are consistent with reports that increases in pH i augment Ca 2+ influx via high-voltage-activated Ca 2+ channels [23,28,47,50], the attenuation of the enhanced Ca 2+ influx by high b T was not complete. This may in part reflect the inability of 100 mM internal tricine to abolish the rise in pH i evoked by a high concentration of TMeA, although effects independent from increases in pH i also likely contribute.…”

“…An additional possibility is that weak-base-induced increases in Ca 2+ influx could in part reflect the inhibition of the medium and slow AHPs, which normally act as a negative feedback mechanism to regulate Ca 2+ influx [11,13,16,30]. This could also provide an explanation for previous findings that externally applied weak bases have a greater effect to augment Ca 2+ -dependent depolarizing potentials (evoked under conditions where AHPs were not blocked) than I Ca (evoked under conditions where all K + currents were blocked) [47] (see also [50]). Inhibition of mI AHP and sI AHP , with a consequent increase in Ca 2+ influx and decrease in K + efflux, may also contribute to the development of the ADPs occasionally seen in the presence of NH 3 or TMeA [36,56,57].…”

pH Regulation in Horizontal Cells of the Skate Retina

“…6(B) yields an intrinsic β i value at the horizontal cell's resting pH i of 24n2 m\pH unit. This value is within the range of measurements that have been made in neurons from a variety of invertebrates, 9-33 m\pH unit (Chesler, 1990), and not much higher than the intrinsic buffer capacity (10-15 m\pH) estimated in catfish horizontal cells (Dixon et al, 1993).…”

“…For example, experimentally-induced changes in extracellular pH have been shown to modify synaptic transmission from photoreceptors to second-order neurons (Barnes, Merchant and Mahmud, 1993 ;Harsanyi and Mangel, 1993), and recent evidence indicates that -glutamate, the neurotransmitter released by vertebrate photoreceptors, raises the intracellular proton concentration of catfish horizontal cells (Dixon, Takahashi and Copenhagen, 1993). The resultant acidification has been shown to suppress the high voltage-activated calcium current of these second-order neurons (Takahashi, Dixon and Copenhagen, 1993), which can, in turn, affect both the membrane potential and the calcium-dependent process of transmitter release. On the other hand, a voltage-dependent inwardly-rectifying K + current was found to be relatively insensitive to acidification, but alkalinization shifted the activation range and greatly enhanced the current responses induced by hyperpolarizing voltage steps (Takahashi and Copenhagen, 1995).…”

Calcium regulation in photoreceptors

“…In addition to the sensitivity of I Ca to extracellular pH, studies of L-type calcium channels in other retinal neurons suggest that intracellular pH can modulate photoreceptor Ca 2+ channels as well. Takahashi et al (115) showed that intracellular alkalinization and acidification of catfish horizontal cells enhanced and reduced, respectively, L-type calcium currents. It is very plausible to assume that changes in pH within photoreceptors due to metabolic activity, ATP hydrolysis, regulation of proton extrusion or transport of calcium via PMCA (20) could in turn regulate calcium influx.…”