title: Modulation of Ca2+ current in canine colonic myocytes by cyclic nucleotide-dependent mechanisms.
abstract: Regulation of Ca2+ currents by cyclic nucleotide-dependent mechanisms was studied in circular muscle cells isolated from canine proximal colon. Whole cell Ca2+ currents were recorded at 32 degrees C with the use of amphotericin B-perforated patches. The effects of several agents known to increase levels of adenosine 3',5'-cyclic monophosphate (cAMP) were tested. Vasoactive intestinal peptide (VIP) and isoproterenol (10(-7) to 10(-5) M) increased Ca2+ current in a concentration-dependent manner. Forskolin (10(-7) M) and dibutyryl cAMP (10(-6) to 10(-5) M) also increased Ca2+ current. Higher concentrations of forskolin (10(-6) to 10(-5) M) caused inhibition of Ca2+ current. Low concentrations (10(-5) to 10(-7) M) of dibutyryl cAMP or 8-bromo-cAMP caused concentration-dependent enhancement in Ca2+ current, and these effects were reversible on washout of the cAMP analogues. When the concentration of cAMP analogues was increased (10(-3) to 10(-4) M), we observed inhibition of Ca2+ current similar to the effects of forskolin. Membrane-permeable analogues of guanosine 3',5'-cyclic monophosphate produced exclusively inhibitory effects. The nonspecific protein kinase inhibitor H-7 (up to 60 microM) failed to block the effects of VIP, isoproterenol, and forskolin, and it produced inhibitory effects on Ca2+ current, independent of agonist stimulation. The data suggest that low levels of cAMP may, via phosphorylation by protein kinase A, enhance L-type Ca2+ current, but higher concentrations of cAMP may "cross over" and activate protein kinase G. Phosphorylation by protein kinase G appears to produce a dominant inhibition of Ca2+ current.
authors.affiliation: Department of Physiology and Cell Biology, University of Nevada School of Medicine, Reno 89511, USA.
authors.forename: S D
authors.initials: SD
authors.lastname: Koh
authors.forename: K M
authors.initials: KM
authors.lastname: Sanders
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D003106:Colon / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism*; D004285:Dogs; D017136:Ion Transport / Q000187:drug effects; D007545:Isoproterenol / Q000494:pharmacology; D009130:Muscle, Smooth / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology
chemical_list: D000316:Adrenergic alpha-Agonists; D005576:Colforsin; D014660:Vasoactive Intestinal Peptide; D003994:Bucladesine; D000242:Cyclic AMP; D007545:Isoproterenol; D002118:Calcium