[Does guanabenz have a non-specific effect on spontaneous contractions of isolated guinea pig atria?]

Guanabenz was found to produce a concentration-dependent depression of the isometric contractility of the isolated, spontaneously beating atria of the guinea-pig. It also depressed the atrial rate of the isolated, spontaneously beating atria of the guinea-pig. The effect of the increasing concentrations of guanabenz on the heart rate was weaker than its effect on the isometric contraction. A time-dependent depression of both the isometric contraction and of the atrial rate after the addition of a single dose of guanabenz was also found up to 10th min. Guanabenz did not change the maximal driving (following) frequency of the atria. Aminophylline partially, isoprenaline almost completely and calcium completely antagonized the negative inotropic action of guanabenz. They, however, did not antagonize the negative chronotropic action of guanabenz. It seems, regardless of what the precise mechanism(s) of action of guanabenz may be (probably nonspecific on the isolated guinea-pig atria), that all these substances (aminophylline, isoprenaline and calcium) restore the contractility of the isolated atria by compensating the calcium balance which has been previously changed by guanabenz.