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Further observations on the behaviour of ouabain‐insensitive sodium efflux towards proctolin in barnacle muscle fibres.

1. A further study has been made of the stimulatory action of proctolin on the ouabain‐insensitive Na+ efflux in single muscle fibres from the barnacle, Balanus nubilus. 2. (i) Strontium (Sr2+) behaves as a substitute for external Ca2+. In this case, however, the response to proctolin fails to decay. (ii) Injection of Sr2+ stimulates the ouabain‐insensitive Na+ efflux. This effect is mimicked by injecting Ca2+. 3. Depolarization of the fibre membrane with 30 and 100 mM‐external K+ augments the response to proctolin. 4. Pre‐injection of GTP or Gpp(NH)p (sodium 5‐guanylylimidodiphosphate) prevents the response to proctolin from completely decaying. 5. Pre‐injection of guanine nucleotides in conjunction with membrane depolarization stops the response to proctolin from decaying. 6. Measurements of Em before and during treatment with proctolin indicate a prompt but small and reversible fall in the membrane potential. 7. (i) The aequorin response of fibres pre‐treated with ouabain to proctolin is monophasic or multiphasic, and concentration dependent, the minimal effective concentration being in the nanomolar range. (ii) The duration of these signals is usually less than 5 min; this is about half the time it takes for the stimulated Na+ efflux to reach a maximum. (iii) The aequorin response to proctolin occurs quite often in fibres suspended in nominally Ca2(+)‐free artificial sea water. (iv) Sudden graded elevations in external K+ following complete decay of the aequorin response to proctolin are rapidly followed by stepwise transitory increments in light emission. (v) The aequorin response to 100 mM‐external K+ is frequently a triplet. 8. (i) Together, these results are in line with the view that the action of proctolin on the ouabain‐insensitive Na+ efflux is the result of a temporary fall in internal pCa and that its point of action is the Ca2+ channel, where a putative G protein in the presence of GTP or Gpp(NH)p is able to maintain constancy of the hormonal effect. (ii). They strengthen the argument that the barnacle muscle fibre as a preparation is especially suitable for studies of this kind.