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), ST 1059 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.