Effects of midodrine, ST 1059, methoxamine and glycine on spontaneously beating guinea-pig atria.

The chronotropic effects of midodrine, glycine (10(-8) to 3.10(-3) M), ST1059 and methoxamine (10(-8) to 10(-3) M) were investigated in the spontaneously beating guinea-pig right atrial preparation. Midodrine and glycine produced a slight, but significant rise in atrial rate over a wide concentration range. The midodrine-induced rise in atrial rate was not influenced by the beta-adrenergic receptor blocking drug propranolol (10(-6) M). The histamine (H2)-receptor blocking drug metiamide (3.10(-5) M) abolished the positive chronotropic actions of both midodrine and glycine. No positive chronotropic effect was seen after the administration of ST 1059 or methoxamine. A decrease in atrial rate was elicited by high concentrations (above 10(-4) to 10(-3) M) of the sympathomimetic agents midodrine, ST 1059, and methoxamine, but not by the amino acid glycine.