Frequency-Force Relationship on Isolated Rat and Guinea Pig Atria. Effects of Cholinergic and Adrenergic Receptor Antagonists 1

Summary

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.