Developmental acquisition of Ca²⁺-sensitivity by K⁺ channels in spinal neurones

Abstract

A developmental change in the ionic basis of the inward current of action potentials has been observed in many excitable cells¹. In cultured spinal neurones of Xenopus, the timing of the development of the action parallels that seen in vivo^2,3. In vitro, as in vivo, neurones initially produce action potentials of long duration which are principally Ca-dependent; after 1 day of development the impulse is brief and primarily Na-dependent^3–5. At both ages, however, both inward components are present^4–6 and the mechanism underlying shortening of the action potential is unknown. One possibility is that the outward currents change during development. Using the patch-clamp technique⁷, we have recorded single K⁺-channel currents in membrane patches isolated from the cell bodies of cultured embryonic neurones³. The unitary conductance of one class of K⁺ channels was ∼155 pS and depolarization increased the probability of a channel being open. Neither conductance nor voltage dependence seemed to change with time in culture; in contrast, the Ca²⁺-sensitivity of this K+ channel increased. In younger neurones, Ca²⁺-sensitivity was greatly reduced or absent, whereas in more mature neurones, the activity of this channel was Ca-dependent. Such a change could account for the shortening of the action potential duration by increasing the relative contribution of outward currents.