Figure 7.

Competition between calmodulin and the endogenous modulator in a cone outer segment membrane patch. Currents were activated with voltage steps from 0 to −40 mV in the presence of 20 μM Ca⁺⁺ and 100 μM Mg⁺⁺. Shown are difference currents measured by subtracting from currents measured at 60 μM cGMP those measured in the absence of the cyclic nucleotide. The leak conductance, measured in 0 cGMP at −40 mV was 325 pS. Currents in A were measured shortly after excision in the absence (thick trace) or continuous presence (thin trace) of 200 nM calmodulin. Currents in B were measured after exposing the same membrane patch to the EDTA/EGTA solution. Again, currents were measured in the absence (thick trace) or continuous presence (thin trace) of 200 nM calmodulin. Calmodulin is completely ineffective before the endogenous modulator is removed. If the endogenous modulator is first removed by exposure to the EDTA/EGTA solution, calmodulin then causes a shift in membrane current similar in direction, but smaller in extent than that caused by the endogenous modulator.