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

Simultaneous recordings of membrane potential and force, and cytoplasmic calcium ([Ca²⁺](i)) and force were made in rat anococcygeus to determine whether membrane hyperpolarisation plays a role in cyclic nucleotide-induced relaxation. In the presence of phenylephrine (0.2 μM), which evoked sustained contraction, an elevation in [Ca²⁺](i), and depolarisation, nitroprusside (5 μM) caused 96±3% relaxation, 77±3% decrease in suprabasal [Ca²⁺](i), and 16±2 mV hyperpolarisation. Forskolin (1 μM) caused 98±1% relaxation, 92±2% decrease in suprabasal [Ca²⁺](i), and 18±1 mV hyperpolarisation. These responses persisted in the presence of a variety of K⁺ channel blockers or in ouabain. The decrease in [Ca²⁺](i) preceded the 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 hyperpolarisation, which reflects a decline in the Ca²⁺ dependent conductance(s) activated by phenylephrine. Copyright (C) 1998 Elsevier Science B.V.