Calcium dependence of baclofen- and GABA-induced depression of responses to transmural stimulation in the guinea-pig isolated ileum.
Both baclofen and gamma-aminobutyric acid (GABA) induced a dose-dependent depression of cholinergic twitch contractions in the transmurally stimulated guinea-pig isolated ileum. Over a range of 0.6-2.4 mM Ca2+, the degree of depression was inversely related to the Ca2+ concentration, with an increased sensitivity and sinistral shift of the dose-response curve at lower Ca2+ concentrations. Partial occupation of Ca2+ channels by Ruthenium Red (0.1 microM) also potentiated the depressive responses to baclofen and GABA. It is concluded that these agonists, acting through GABAB receptors, limit the availability of Ca2+ required for neurotransmitter release in myenteric motor nerves.
D000818:Animals; D001418:Baclofen / Q000494:pharmacology*; D002118:Calcium / Q000502:physiology*; D004558:Electric Stimulation; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects* / Q000502:physiology; D012430:Ruthenium Red; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
D007473:Ion Channels; D012430:Ruthenium Red; D005680:gamma-Aminobutyric Acid; D001418:Baclofen; D002118:Calcium