Abstract
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.
Footnotes
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Send reprint requests to: Dr. Jerry M. Farley, Department of Pharmacology and Toxicology, University of Mississippi Medical Center, 2500 North State Street, Jackson, MS 39216.
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↵1 This work was supported by the National Institutes of Health grant HL55547 and the Mississippi Lung Association.
- Abbreviations:
- ACh
- acetylcholine
- ANOVA
- analysis of variance
- BKCa
- large-conductance calcium-activated K+ channels
- 8-Br-cGMP
- 8-bromoguanosine 3′,5′-cyclic monophosphate
- [Ca++]i
- intracellular calcium concentration
- ClCa
- calcium-activated Cl−channels
- DMSO
- dimethyl sulfoxide
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- IP3
- inositol 1,4,5-trisphosphate
- KCa
- calcium-activated K+ channels
- PLC
- phospholipase C
- PSS
- physiological saline solution
- STOC
- spontaneous transient outward currents
- VDCC
- voltage-dependent calcium channels
- Received June 30, 1997.
- Accepted January 29, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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