Effect of relaxin on bound cAMP in rat uterus.

Relaxin was able to elevate uterine bound cAMP in the presence of 3-isobutyl-1-xanthine in buffers of low or high (0.5 M NaCl) ionic strength. Significant elevation was observed 10 min after exposure to the hormone. The peak in bound cAMP preceded that in total cAMP (15 vs 20 min, respectively, under isometric conditions). The response was concentration-dependent, with maximal elevation in bound cAMP observed at 0.1-0.4 microgram/ml relaxin. The effect of relaxin on bound cAMP represented a change in a sizable proportion of the total available binding sites. However, relaxin did not elevate bound cAMP in the absence of phosphodiesterase inhibitors under conditions where it clearly inhibited contractile activity. These data suggest that although relaxin can alter the level of bound cAMP in the rat uterus under some conditions, these effects may not be involved in the acute changes elicited by this hormone which result in the rapid inhibition of spontaneous contractile activity.