ISSN:
1432-1351
Keywords:
Limulus photoreceptor
;
Light-induced current
;
Selective block
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Abstract Light-activated receptor currents were measured in Limulus ventral nerve photoreceptors by a two electrode voltage clamp. Short flashes stimulate three different current components (C1, C2, C3). Currents were measured before and after injection of neomycin, heparin and BAPTA. These substances inhibit the inositol phosphate pathway at different stages. Each substance selectively blocked the C2 component. After the block of C2 the other two components could be further stimulated, but their amplitudes were usually reduced. High concentrations of BAPTA, however, increased the amplitude (two-fold) and decay time constant (eight-fold) of C1, suggesting that calcium is necessary for the deactivation of the C1 current. The time to maximum for C2 is known to be dependent on the light-adaptation state and the stimulus intensity. It is shown here that the time to the maximum of C2 remains nearly unchanged when C2 is gradually inhibited by BAPTA, but is prolonged when C2 is inhibited by neomycin. Since the kinetics of an enzyme reaction depend on the substrate concentration, these results indicate, as expected, that neomycin changes the substrate concentration and thus changes the rate and gain of the cascade, while BAPTA changes the product concentration, i.e. binds the released calcium. Therefore, these observations support the hypothesis that the negative feedback of adaptation regulates the phospholipase C cascade at the same early stage, where the neomycin acts. The same consideration suggests that after the calcium release no further amplification occurs in the transduction cascade since the time to maximum with BAPTA remains unchanged. The results are explained by three parallel transduction pathways. It is suggested that the inositol phosphate cycle regulates the gain of the other two pathways by the release of calcium ions.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00207189
Permalink
