Vol. 285, Issue 2, 588-594, May 1998

Effects of 8-Bromo-Cyclic GMP on Membrane Potential of Single Swine Tracheal Smooth Muscle Cells¹

Jaehwa Choi and Jerry M. Farley

Department of Pharmacology and Toxicology, University of Mississippi Medical Center, Jackson, Mississippi

Cyclic GMP relaxes swine tracheal smooth muscle. Relaxation occurs because of decreases in intracellular calcium concentration ([Ca⁺⁺]_i) that are thought to occur through hyperpolarization which inhibits calcium influx. Activation of K⁺ channels has been suggested as the underlying mechanism for the hyperpolarization. In the present study, the effects of 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP, a membrane-permeable analog of cyclic GMP) on acetylcholine (ACh)-induced increases in [Ca⁺⁺]_i were examined by laser scanning confocal microscopy in fluo 3-loaded single cells. Membrane potential and currents were measured by the perforated-configuration of patch-clamp method. 8-Bromo-cGMP (1 µM-0.1 mM) inhibited 0.1 µM ACh-induced oscillations in [Ca⁺⁺]_i in a concentration-dependent manner. Spontaneous changes in membrane potential were observed by the patch-clamp method. Acetylcholine (0.03 µM) did not affect the time-averaged mean potential. The spontaneous changes in membrane potential were reduced and the cells were depolarized by 0.1 µM ACh and to a greater degree by 1 µM ACh. This result is consistent with previous observations of ACh-induced depolarization in intact tissue. The application of 0.1 mM 8-Br-cGMP had no significant effects on spontaneous changes in membrane potential and did not induce changes in membrane potential in cells treated with 0.1 µM ACh. In voltage-clamped cells, ACh (0.1 µM) induced oscillations in calcium-activated K⁺ currents. 8-Bromo-cGMP (0.1 mM) inhibited these ACh-induced oscillations in currents, but had no significant effects on spontaneous changes in membrane current in unstimulated cells. These data indicate that 8-Br-cGMP inhibits ACh-induced increases in [Ca⁺⁺]_i by mechanisms other than regulation of membrane potential.