Pharmacological Actions of 1- (7-indenyloxy) -3-isopropylaminopropan-2-ol hydrochloride (YB-2) in Isolated Rat Heart Muscles

The pharmacological actions of 1-(7-indenyloxy)-3-isopropylaminopropan-2-ol hydrochloride (YB-2) in isolated rat heart muscles were examined on (a) the spontaneous contraction rate and tension of the right atrium, (b) the effect on arrhythmic contractions of the right atrium induced by the electrical stimulation, (c) the contractile tensions of the electrically driven papillary and left atrial muscles, (d) the refractory periods of the papillary and left atrial muscles, and (e) the correspondences of responses to high frequency stimulation in the papillary and left atrial muscles. Materials and MethodsAdult rats of both sexes weighing 300 to 400g were used. After sacrifice by severing the common carotid arteries, the heart was excised. The right and papillary muscles were prepared and severally suspended in a Magnus' apparatus in a Locke's solution saturated with pure oxygen at 30 to 31°C.The right atrium was used to examine the drug effects on rate and tension of spontaneous contractions and on their electrically induced arrhythmic contraction, while the papillary and left atrial muscles were used to examine the drug effects on their contractile tensions, refractory periods, and correspondences of responses to high frequency stimulations. When the papillary and left atrial muscle tissue were used, they were driven by electrical stimulations at supramaximal intensities, at durations of 10 and 5 msec, respectively, and 120 cycles per min. The drug applied was added to the medium in which the preparations were contracting 30 min before the estimation of each drug effect, and was expected to act easily on these tissues. When the drug effect on the arrhythmic contraction of the right atrial spontaneous contractions induced by electrical stimulation was examined, the stimulations were given to the preparation at critical intensities, at a duration of 5 msec, and at 120 cycles per min, and, then, it was examined whether the electrically induced arrhythmic contractions could be prevented by the drug or could not.When the refractory periods of the papillary and left atrial muscles were examined, Govier's method was used: basal stimulations were given to the muscles at supramaximal intensities, at durations of 10 and 5 msec, respectively, and at 60,120, and 240 cycles per min;and test stimulations, whose conditions were the same as those of the basal stimulation. If the time intervals between the two kinds of stimuli were of shorter durations than the refractory periods proper to the tested muscles, muscular contractile responses corresponding to the test stimulations would hardly appear. When the muscular responses corresponding to the test stimulation began to appear during gradual increase of the time intervals, the estimated time intervals were, defined to represent the refractory periods of the muscle tested.