Physiological effects of a novel bioactive agent

Urinary bladder smooth muscle contraction can be evaluated using field stimulation (neurohumoral transmission), carbachol (muscarinic stimulation), and KCl (direct membrane depolarization). We recently evaluated the activity of a novel organic chemical, macrocycle-1, on the contractile responses of the bladder to field stimulation, carbachol, and KCl. Isolated strips of rabbit bladder were mounted in individual baths containing 7.5 ml Tyrode's solution. The response to FS (1-32 Hz), carbachol (1 mumol/l), and KCl (120 mmol/l) were determined in the presence and absence of 3 different concentrations of macrocycle-1. Maximal tension, the rate of tension generation, the time to maximal tension, and the rate of decay following maximal tension were determined. The results can be summarized as follows: (1) In the absence of macrocycle-1, maximal tension and the maximal and mean rate of tension generation increased with frequency, whereas the time to maximal tension was constant. The rate of decay of tension following maximum tension was greater for 8, 16 and 32 Hz as compared to 1 or 2 Hz. (2) The maximal response to KCl was lower than either FS or carbachol. The maximal rates of tension generation for carbachol and KCl were lower than that of FS; and the rate of tension generation for KCl was lower than that of carbachol. The time to maximal stimulation for KCl was greater than that of either carbachol or FS. (3) Macrocycle-1 had a greater inhibitory effect on KCl stimulation than on carbachol stimulation; and a greater inhibitory effect on KCl and carbachol stimulation than on FS. (4) The rate of tension generation was more sensitive to macrocycle-1 inhibition than was the maximal tension responses to all methods of stimulation. Our current hypothesis is that macrocycle-1 is acting as an intracellular calcium buffer whose affinity constant and association rate does not interfere with rapid intracellular release mechanisms (FS) while it inhibits slow intracellular calcium release mechanisms (carbachol and KCl).