Differences in antagonist affinities at muscarinic receptors in chick and guinea-pig.

1. The antimuscarinic effects of a number of muscarinic antagonists in left atrium and ileum of the chick and guinea-pig were evaluated. 2. In agreement with findings in the guinea-pig, pirenzepine did not differentiate between muscarinic receptors in the left atrium and ileum of the chick. However, its affinity in this species was 5- to 10-fold higher than that observed in similar mammalian tissues. 3. Gallamine, methoctramine and himbacine are cardioselective antagonists in the guinea-pig with a selectivity index of 20, 25 and 10 respectively. However, in the chick, while himbacine maintained its cardioselective effect, gallamine was non-selective and methoctramine was only 4-fold more selective. 4. Secoverine, a non-selective antagonist in guinea-pig exhibited a small degree of cardioselectivity in the chick. 5. The ileoselective effect of 4-DAMP in the chick was much greater than that observed in the guinea-pig. 6. It is concluded that muscarinic receptors in cardiac and smooth muscles of the chick differ from those in the guinea-pig.