Effect of isoprenaline on Ca2+ channel current in single smooth muscle cells isolated from taenia of the guinea-pig caecum.
1. The effects of isoprenaline (Iso) on Ca2+ channel current in enzymatically isolated single cells of the guinea-pig taenia caeci were examined using the standard whole-cell voltage-clamp method. 2. Iso potentiated the voltage-dependent Ca2+ current; the threshold and maximally effective concentration of Iso to increase Ca2+ current were 3-10 nM and 1-3 microM, respectively. The average increase in Ca2+ current produced by 3 microM Iso was 42 +/- 6% (mean +/- S.E.M.) and the response could be obtained repeatedly in the same cell. The concentration-response relationship could be fitted by a binding model with a Hill coefficient of 1 and a dissociation constant of 42 nM. 3. The effect of Iso on Ca2+ current was voltage dependent. Although potentiation of Ca2+ current by Iso was obvious between -30 and +10 mV, it was small or absent around +20 to +30 mV. Iso had little effect on the relationship between inactivation of the Ca2+ current and voltage obtained using a double-pulse protocol. 4. External application of forskolin, an adenylyl cyclase activator, or internal perfusion of cAMP or dibutyryl cAMP from the recording pipette, did not increase Ca2+ current and potentiation of Ca2+ current by Iso was observed repeatedly and was unchanged. 5. Internal perfusion of GTP gamma S or GDP beta S increased or did not affect the Ca2+ current and potentiation of Ca2+ current by Iso was unchanged and could be recorded repeatedly for about 20 min after rupture of the cell membrane. In addition, treatment of cells with the potent protein kinase C inhibitor, chelerythrine, had no effect on Ca2+ current or on potentiation of Ca2+ current by Iso. 6. These results suggest that the Ca2+ current in guinea-pig taenia caeci cells is potentiated by isoprenaline via mechanisms which do not involve either a cAMP pathway, a G-protein pathway or a protein kinase C pathway. The receptor involved appeared to be an atypical adrenoreceptor not blocked by either alpha- or beta-receptor blocking agents.
Muraki
K
K
Department of Pharmacology and Clinical Pharmacology, St George's Hospital Medical School, London.
Bolton
T B
TB
Imaizumi
Y
Y
Watanabe
M
M
D000818:Animals; D015220:Calcium Channels / Q000187:drug effects* / Q000378:metabolism; D002432:Cecum / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006153:Guanosine Diphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D009130:Muscle, Smooth / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D011941:Receptors, Adrenergic / Q000187:drug effects / Q000502:physiology; D013873:Thionucleotides / Q000494:pharmacology
D015220:Calcium Channels; D011941:Receptors, Adrenergic; D013873:Thionucleotides; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C023427:guanosine 5'-O-(2-thiodiphosphate); D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D007545:Isoproterenol