Effects of cooling on the contractile response of the rat tracheal muscle to pilocarpine in vitro.

Experiments were designed to determine the effect of cooling on the contractile response of rat tracheal strip-chain preparation to the partial agonist pilocarpine. The preparation was suspended in an organ bath containing Krebs bicarbonate solution for isometric tension recording. A decrease of the bath temperature from 37 to 30, 20 or 10 degrees C (cooling) did not affect or inhibited the contractile response of the trachea caused by pilocarpine (1-100 microM), while it augmented the carbachol (0.1-1 microM)-induced response. At 37 degrees C, the pilocarpine (3-100 microM)-induced contraction was relatively resistant to removal of extracellular Ca2+ or to the Ca2+ entry blocker, verapamil (1 microM). Similar results were also obtained for the carbachol (0.3-10 microM)-induced response. On the other hand, the affinity of pilocarpine for the tracheal muscarinic receptors, determined from its dissociation constant (KA), was significantly decreased at a lower temperature. It is concluded from these observations that cooling-induced subsensitivity of the rat tracheal muscle to pilocarpine is mainly associated with a decrease in the affinity of the agonist for muscarinic receptors.