Frequency-force relationship on isolated rat and guinea pig atria. Effects of cholinergic and adrenergic receptor antagonists.

Abstract

The influence of several rates of contractions (electrically induced by means of two different experimental designs, named as type A and type B) on the isometric developed tension amplitude (IDTA) of left auricles isolated from rats and from guinea pigs were studied. Furthermore, the frequency-force relationship of comparable preparations in the presence of cholinergic or adrenergic alpha or beta receptor antagonists (atropine, H 56/28, or phentolamine) was also explored. Procedure type A stimulation consisted of different rates of stimulation (25, 50, 100, and 200/min) applied stepwise to each atrium, whereas with procedure type B, each atrium was stimulated at only one of the above-mentioned frequencies. The IDTA of rat atria stimulated by procedure type A decreased stepwise with the increment of frequency [negative or descending staircase (N or DS)], whereas following procedure type B it was comparable at all the frequencies studied. On the contrary, the IDTA of guinea pig atria augmented stepwise with the increments in the stimulatory rate [positive or ascending staircase (P or AS)] either following procedure type A or type B. Atropine abolished the N or DS of isolated rat atria. In the presence of H 56/28 or phentolamine, a P or AS in guinea pig auricles was still observable; however, after a combined treatment with these two antagonists of beta and alpha adrenergic receptors, a complete blockade of the P or AS was obtained. It would appear that a cholinergic mechanism is involved in the N or DS of isolated rat atria, whereas an adrenergic one could be associated with the P or AS of guinea pig auricles. Furthermore, the appearance of the N or DS in isolated rat atria depends on the procedure of stimulation applied to the tissue.