Comparison of time constants of single channel patches, quantum bumps, and noise analysis in Limulus ventral photoreceptors.

The characteristic time constants derived from three different experimental procedures for measuring light-evoked currents in photoreceptors are compared; these procedures include single-channel patch-clamp measurements, noise analysis, and current relaxation studies. Recent patch-clamp measurements of the mean open times of single light-activated channels in the ventral photoreceptor of Limulus (Bacigalupo, J., Lisman, J.E. (1983), Nature (London) 304:268-270) yield a disagreement of the measured mean open time with the relaxation time of the falling phase of quantum bumps and with the inverse characteristic frequency of the noise power spectrum, measured by Wong (Wong, F. (1978), Nature (London) 276:76-79). We present new experimental results which show that the relaxation time of the falling phase of bumps is markedly shortened by light-adaptation. Hence the state of light-adaptation has to be taken into account when comparing different experiments. Secondly, we investigate three simple models for the mechanism of channel opening and closing, and conclude that an agreement of the mean open time of single channels, the relaxation time of the falling phase of bumps, and the inverse characteristic frequency of the noise power spectrum cannot be expected.