[ABSTRACT]
1. The outer segment of an isolated rod photoreceptor from the bullfrog
retina was drawn into a pipette containing choline solution for recording
membrane current. The rest of the cell was sheared off with a glass probe
to allow internal dialysis of the outer segment with a bath potassium solution
('truncated rod outer segment' preparation). The potential between the
inside and the outside of the pipette was held at 0 mV.
2. Application of bath cGMP, in the presence of 3-isobutyl-1-methylxanthine
(IBMX), gave rise to an outward membrane current. At saturating cGMP concentrations,
this current was insensitive to intracellular Ca2+ at concentrations between
0 and 10 microM. At subsaturating cGMP concentrations, however, this current
was inhibited by intracellular Ca2+. This sensitivity to Ca2+ declined
after dialysis with a low-Ca2+ solution, suggesting the involvement of
a soluble factor.
3. At low (nominally 0) Ca2+, the half-maximal activation constant
and Hill coefficient for the activation of the cGMP-gated current by cGMP
were 27 microM and 2.0, respectively. At high (ca 10 microM) Ca2+, the
corresponding values were 40 microM cGMP and 2.4.
4. The inhibition of the current by Ca2+ was characterized at 20 microM
cGMP. Ca2+ inhibited the current by up to 60%, with half-maximal inhibition
at 48 nM Ca2+ and a Hill coefficient of 1.6.