Abstract
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.