We have studied the current-carrying ability and blocking action of various divalent cations in the Ca channel of Lymnaea stagnalis neurons.Changing the concentration or species of the permeant divalent cation shifts the voltage dependence of activation of the Ca channel current in a manner that is consistent with the action of the divalent cation on an external surface potential . Increasing the concentration of the permeant cation from 1 to 30 mM produces a twofold increase in the maximum Ca current and a fourfold increase in the maximum Ba current; the maximum Ba current is twice the size of the maximum Ca current for 10 mM bulk concentration. Correcting for the changing surface potential seen by the gating mechanism, the current-concentration relation is almost linear for Ba t+ , and shows only moderate saturation for Ca" ; also, Ca t+ , Bat+ , and Sr" are found to pass through the channel almost equally well . These conclusions are obtained for either of two assumptions : that the mouth of the channel sees (a) all or (b) none of the surface potential seen by the gating mechanism . Cd2 + blocks Lymnaea and Helix Ca channels at concentrations 200 times smaller than those required for Cot+ or Nit+ . Ca 21 competes with Cd 2 + for the blocking site ; Bat+ binds less strongly than Ca 21 to this site . Mixtures of Ca2' and Ba t+ produce an anomalous mole fraction effect on the Ca channel current . After correction for the changing surface potential (using either assumption), the anomalous mole fraction effect is even more prominent, which suggests that Bat+ blocks Ca current more than Ca2' blocks Ba current .

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Indirect flight muscle (IFM) contracts at high frequencies at a priming level of Ca2þ that stays constant during oscillations. The muscles are stretch-activated. Alternating contraction of opposing muscles produces resonant distortions of the thorax, which results in rapid movement of the wings. The TnC isoform, F1, which binds one Ca2þ in the C-lobe, is needed for stretch-activation. The N-lobe of F1 is inactive and does not bind TnH (the TnI of IFM). The C-lobe changes from a closed to open conformation on binding Ca2þ. However, the binding of TnH to this lobe is independent of Ca2þ, and the transition may be necessary for optimum orientation of TnH. The minor TnC isoform, F2, which is needed for the development of isometric force at relatively high [Ca2þ], binds one Ca2þ in each lobe; association with TnH involves both lobes and is Ca2þ-dependent. In Lethocerus IFM, the C-terminus of TnH is close to a crossbridge, and may form part of a ''troponin bridge'' between the thick and thin filament, transmitting strain to the thin filament on stretch. The inhibitory sequence of TnH would be pulled off actin by stretching, rather than by reversibly binding to the N-lobe of TnC in the presence of Ca2þ, as occurs in skeletal muscle. We will describe the effect of the C-terminal half of F1 alone on oscillatory contraction, and the effect of adding the C-terminal region of TnH to compete with endogenous TnH.

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