Determining the parameters of concentration-response relationship for histamine on the isolated portion of guinea pig stomach and guinea pig right atrium

In our previous experiments measuring acid secretion in the isolated mouse stomach we found higher EC50 values (agonist concentration, evoking half ma ximal effect) for histamine than in the experiments on the isolated guinea pig right atrium (contraction frequency). The difference could be due to no nequilibrium conditions in the isolated mouse stomach preparation or to spe cies differences. In order to check this possibility and to test the consis tency of concentration-response relationship (CRR) parameter determination on an isolated secretory organ, we compared EC50 for histamine and pA(2) fo r famotidine on the two isolated preparations from the same species, guinea pigs. Both, EC50 and pA(2) values were significantly different when measuring gas tric acid secretion (9.7 mu M and 6.07) from the parameters, found in the i solated right atrium measuring frequency of contractions(1.6 mu M and 7.06) . From these results it can be concluded that the isolated portion of guinea pig stomach is suitable for CRR determination; but the parameters obtained are in the range of higher concentrations than the real ones bemuse of kine tic particularities of the preparation.