Tsai_1989_Cryobiology_26_485

Lowering temperature from 37 degrees C to 22, 18, and 14 degrees C triggered automaticity of smooth longitudinal muscle of guinea pig isolated ileum. The amplitude of the hypothermia-induced automaticity was dependent on the degree of temperature drop: the greater the temperature drop, the greater the amplitude. However, when the preparation was initially prepared and maintained at 14 degrees C and then the temperature was raised at a similar rate to 18, 22, and 37 degrees C, the automaticity was not observed. This series of observations suggests that cooling rate may be the trigger and/or part of the triggering mechanism for the observed automaticity. Mepenzolate (1.0 x 10(-6) M), a specific muscarinic receptor antagonist, blocked the automaticity suggesting the involvement of muscarinic receptors in the pathogenesis and/or the manifestation of the automaticity. Verapamil (1.0 x 10(-7) M), a calcium channel blocker which inhibits the transmembrane Ca2+ influx into smooth muscle cells during excitation, blocked the automaticity suggesting that transmembrane Ca2+ influx plays a significant role in the pathogenesis and/or manifestation of the automaticity. A specific cytoplasmic calcium channel blocker, 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride (1.0 x 10(-6) M) blocked the automaticity, suggesting that cytoplasmic calcium also plays a significant role in the pathogenesis and/or manifestation of the automaticity. In order to characterize the temperature-induced changes in the muscarinic receptors, an attempt was made to use the classic method of Furchgott and Burstyn to determine the dissociation constants of acetylcholine at muscarinic receptors at different temperatures. However, the alkylation of muscarinic receptors with phenoxybenzamine at lower temperatures was erratic and the recovery from the occlusion was too rapid to apply the method of Furchgott and Burstyn. We concluded that the lack of reversibility of the effects of phenoxybenzamine at 37 degrees C is due to the predominance of covalent bonding of phenoxybenzamine with the receptors, whereas at lower temperatures like 24 degrees C, the blockade of the muscarinic receptors by phenoxybenzamine is mainly due to simple occlusion.