Force, membrane potential and cytoplasmic Ca2+ responses to cyclic nucleotides in rat anococcygeus muscle

Simultaneous recordings of membrane potential and force, and cytoplasmic ca lcium ([Ca2+](i)) and force were made in rat anococcygeus to determine whet her membrane hyperpolarisation plays a role in cyclic nucleotide-induced re laxation. In the presence of phenylephrine (0.2 mu M), which evoked sustain ed contraction, an elevation in [Ca2+](i), and depolarisation, nitroprussid e (5 mu M) caused 96 +/- 3% relaxation, 77 +/- 3% decrease in suprabasal [C a2+](i), and 16 +/- 2 mV hyperpolarisation. Forskolin (1 mu M) caused 98 +/ - 1% relaxation, 92 +/- 2% decrease in suprabasal [Ca2+](i), and 18 +/- 1 m V hyperpolarisation. These responses persisted in the presence of a variety of K+ channel blockers or in ouabain. The decrease in [Ca2+](i) preceded t he commencement of relaxation whereas the onset of hyperpolarisation lagged behind. Thus, cyclic nucleotide-mediated relaxation in rat anococcygeus is not dependent on hyperpolarisation mediated by the opening of K+ channels. Rather, it is suggested that the decrease in [Ca2+](i) gives rise to hyper polarisation, which reflects a decline in the Ca2+ dependent conductance(s) ;activated by phenylephrine. (C) 1998 Elsevier Science B.V. All rights rese rved.