Calcium and sodium dependence of the biphasic response of the guinea-pig ileum to agonists.
The isometric responses of the longitudinal smooth muscle of the guinea-pig ileum to acetylcholine and to histamine consist of a phasic contraction followed by a fade and a tonic contraction. At high agonist concentrations the fade is more pronounced and a period of lower tonus is observed in the early stage of the tonic component of the response. Experiments done in Ca2+-free medium indicate that the events responsible for the shape of the response take place in the absence of Ca2+ and of contraction. The phasic contraction was inhibited by hyperpolarization in low-Na+ medium. Small decreases in the external Na+ concentration caused a diminution of the fade. Reduction of the Na+ concentration during the early stages of the tonic response caused contraction whereas relaxation was observed at the late stages. It is proposed that the fade is dependent on a Na+-sensitive mechanism that is predominant at early but not at late stages of the responses to agonists.
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D005260:Female; D006168:Guinea Pigs; D006632:Histamine / Q000494:pharmacology; D007082:Ileum / Q000187:drug effects; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects*; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000502:physiology*; D013997:Time Factors
D006632:Histamine; D012964:Sodium; D000109:Acetylcholine; D011188:Potassium; D002118:Calcium