Ionic dependence of early adaptation in the crustacean stretch receptor

In the present study we have examined the effects of changes in potassium and calcium concentration on the early adaptation of the slowly adapting stretch receptor of the crayfish using intracellular recordings including the potential clamp technique. This was because previous studies had suggested that the early adaptative decline of the receptor potential may be attributed mainly to ionic mechanisms involved in the transducer process. During prolonged exposure to K-free saline the cell depolarized; the early adaptive fall of the receptor potential was reduced and finally the response became almost rectangular. These effects developed more rapidly if the concentration of Ca was reduced in the K-free saline. It was shown by injection of current that the effects were not potential dependent. Removal of Ca reduced the amplitude of both the dynamic and static phase of the receptor potential. Isotonic Ca-saline suppressed the static phase of the receptor potential and prolonged exposure completely abolished the response. Potential clamp experiments demonstrated that in the Ca-free saline the passive membrane conductance increased; the static phase of the receptor current increased while the peak current decreased somewhat. In the K-free and Ca-free saline both phases of the receptor current increased. The present results support earlier findings that the major part of the early adaptive fall of the receptor potential is caused by an outward K+ current. Ca2+ modifies the adaptive fall and the static phase, most likely by activation of a Ca2+-dependent K+ current and/or by inactivation of the Na+ current.