Role of calcium in regulating the cyclic GMP cascade of phototransduction in retinal rods.

Abstract

Both cGMP and Ca2' appear to be involved in the process of phototransduction in vertebrate rods, but their precise roles have been the subject of debate. To investigate the role of Ca21 we have artificially increased the calcium buffering capacity of the rod by using a patch pipet to incorporate the calcium buffer 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) into the rod cytoplasm. In the presence of buffer the Na'-Ca2' exchange current became greatly slowed, suggesting that the cytoplasmic calcium concentration (Cai) had indeed been buffered substantially. Although the presence of buffer had negligible effect on the rising phase of the light response, it profoundly altered the later behavior. Responses to brief flashes became prolonged and exhibited an overshoot, apparently because the shut-off process was modified. The normal acceleration of time-to-peak with brighter flashes (an early sign of light adaptation) disappeared. Responses to steady adapting illumination took much longer than normal to settle to a steady level, although the final level represented a similar fractional suppression of current. With superimposed test flashes the presence of such adapting illumination caused a more rapid recovery, whereas the presence of calcium buffer slowed the recovery. The results are consistent with the idea that the rapid drop in Ca1, which has recently been shown to accompany the light response, is involved in terminating the light response, and that Cai is thereby involved in setting the operating point and sensitivity of phototransduction. From comparison with other work we infer that Cai appears to act, at least in part, by means of control of cGMP phosphodiesterase activity.The electrical response of vertebrate photoreceptors to illumination consists of a hyperpolarization (1, 2) brought about by the closure of ion channels in the outer segment plasma membrane (2, 3). The transduction process has very high amplification under dark-adapted conditions, and the absorption of a photon by a single rhodopsin molecule can block the entry into the outer segment of more than 106 Na+ ions (4). In the presence of background illumination the photoreceptor becomes light-adapted, and the gain of transduction decreases in approximately inverse proportion to the light intensity (5, 6), in cones and in rods of lower vertebrates. This automatic gain control is probably of great importance to the operation of the visual system.The need for a diffusible internal messenger substance mediating transduction was recognized from the outset (2, 7), and the proposal was originally made that channel closure was brought about by the release of Ca2+ into the cytoplasm (7). Subsequently the importance of cGMP was recognized, and a cascade of biochemical reactions leading to the activation of a cGMP phosphodiesterase (PDEase) was described; many groups were involved in these developments and the subject is reviewed in refs. 8 and 9. For some years, however, it was not clear whether either or both Ca2l and cGMP we...

Cytoplasmic calcium as the messenger for light adaptation in salamander rods.

“…It is of some interest that the four flash responses in Fig dark level in Ringer solution, since previous experiments have shown that manipulations expected to elevate CaF+ lead to a lengthening of the duration of bright flash responses Matthews, Torre & Lamb, 1985;Hodgkin, McNaughton & Nunn, 1986;Torre et al 1986). Results similar to those in Fig.…”

“…3B, we incorporated the calcium buffer BAPTA into the cytoplasm. The presence of buffer would be expected to slow any changes in Ca2+ and therefore to slow correspondingly the time course of current increase (see, for example, Lamb et al 1986;Torre et al 1986). Figure 3C shows the response in low-Ca2 , 0-Na+ solution (i.e.…”

Gating of cyclic nucleotide-gated channels is voltage dependent

“…Therefore, evolution, through the substitution of two amino acids (YG) in the filter of K + channels and the local modification of the amino-acid sequence of the interface between the voltage-sensing domain and the pore, has transformed a member of the family of voltage-gated channels into a non-voltage-gated channel, but only for Na + and K + . As a consequence of this substitution, CNG channels have also lost selectivity, allowing the permeation of Ca 2 + ions 13 and having a major role in the sensory adaptation in both photoreceptors and olfactory sensory neurons 46,47 .…”

Toward a unified model of vertebrate rod phototransduction

“…One interpretation of the Fain et al result is that the steps cause a gain reduction early in the cascade, in the activation stages (e.g. Torre et al, 1986;Kawamura & Murakami, 1991;Pepperberg et al, 1994;Matthews, 1995;Fain et al, 2001). By this scheme, a saturating flash following a light-adapting step will simply not elicit as many PDE* as a dark-adapted flash, thus reducing T sat .…”

“…Nakatani & Yau, 1988;Forti et al, 1989;Tamura et al, 1991;Koutalos et al, 1995) and late in the recovery to bright flashes (e.g. Baylor et al, 1974;Torre et al, 1986;Pepperberg et al, 1992Pepperberg et al, ,1994.…”