Guinea-pig and rabbit papillary muscles differ in their response to histamine

Guinea-pig and rabbit papillary muscles differ in their response to histamine. 1. Basal developed force (BDF) and maximal developed force (MDF) following isoproterenol and histamine were determined in rabbit and guinea-pig papillary muscles under a variety of experimental conditions. Hypothermia and an increase in the calcium concentration increased the BDF in both tissues. 2. In the guinea-pig MDF increased to a similar extent following both agonists. In the rabbit the MDF response to histamine was much less than the response to isoproterenol. 3. In the guinea-pig papillary muscle, stimulation of both beta- and H2-receptors produces an increase in cyclic AMP. The rabbit papillary muscle contains H1-receptors, stimulation of which has no effect on cyclic AMP. The reduced MDF effect of histamine compared to isoproterenol in the rabbit may be due to the differing biochemical events following beta- or H1-stimulation.