Modulation of voltage-dependent Ba ²⁺ currents in the guinea-pig gastric antrum by cyclic nucleotide-dependent pathways

We have investigated whether the activation of cAMP- and cGMP-dependent pathways modifies the properties of voltage-dependent Ba ²⁺ currents (I _Ba) recorded from guinea-pig gastric myocytes using patch-clamp techniques. All experiments were carried on single smooth muscle cells, dispersed from the circular layer of the guinea-pig gastric antrum. Both dibutyryl cAMP (db-cAMP, 0.1-1 mM), a membrane-permeable ester of cAMP, and isoproterenol, a selective β-stimulant, inhibited I _Ba in a concentration-dependent manner. Forskolin, but not dideoxy-forskolin, an inactive isomer of forskolin, inhibited the peak amplitude of I _Ba. In the presence of either Rp-cAMP or the PKA (cAMP-dependent protein kinase) inhibitor peptide 5-24 (PKA-IP), neither forskolin nor db-cAMP inhibited I _Ba. After establishing a conventional whole-cell recording, the peak amplitude of I _Ba gradually decreased when the catalytic subunit of PKA was included in the pipette. The further application of Rp-cAMP reversibly enhanced I _Ba. Sodium nitroprusside (0.1-1 mM) and 8-Br-cGMP (0.1-1 mM) also inhibited I _Ba in a concentration-dependent manner. The inhibitory effects of forskolin or db-cAMP on I _Ba were not significantly changed by pretreatment with a cGMP-dependent protein kinase (PKG) inhibitor. Similarly, the inhibitory actions of 8-Br-cGMP on I _Ba were not modified by PKA-IP. The membrane-permeable cyclic nucleotides db-cAMP and 8-Br-cGMP caused little shift of the voltage dependence of the steady-state inactivation and reactivation curves. Neither of the membrane-permeable cyclic nucleotides db-cAMP or 8-Br-cGMP had additive inhibitory effects on I _Ba. These results indicate that two distinct cyclic nucleotide-dependent pathways are present in the guinea-pig gastric antrum, and that both inhibited I _Ba in an independent manner.