PT  - JOURNAL ARTICLE
AU  - Leslie D. Napier
AU  - Sandra C. Roerig
AU  - Darice A. Yoshishige
AU  - Barbara A. Barron
AU  - James L. Caffrey
TI  - Canine Cardiac Muscarinic Receptors, G Proteins, and Adenylate Cyclase after Long-Term Morphine
DP  - 1999 Nov 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 725--732
VI  - 291
IP  - 2
4099  - http://jpet.aspetjournals.org/content/291/2/725.short
4100  - http://jpet.aspetjournals.org/content/291/2/725.full
SO  - J Pharmacol Exp Ther1999 Nov 01; 291
AB  - Short-term morphine stimulates vagal bradycardia. This led us to propose the hypothesis that chronically administered morphine would down-regulate myocardial muscarinic receptor systems. Dogs received morphine continuously for 2 weeks through an s.c. catheter, and cellular aspects of parasympathetic control of the heart were examined. Contrary to expectations, morphine increased muscarinic receptor density in the right atrium and left ventricle by 17 and 34%, respectively, with no change in the apparent affinity of the receptor (KD). Morphine also increased the expression of the G protein Giα by 115 and 233%, respectively, in right atrial and left ventricular sarcolemmal membranes. Morphine increased ventricular and atrial Gsα to a much lesser degree (49 and 25%). Morphine failed to alter basal or maximally stimulated (forskolin plus MnCl2) adenylate cyclase activity. The maximum cyclase activation by isoproterenol and the maximum inhibition by carbachol were similarly unaltered by morphine. Morphine reduced the ventricular but not atrial norepinephrine. Both long- and short-term morphine lowered tissue epinephrine content, suggesting that short-term morphine reduces extraneuronal uptake. Potential systemic and cellular models for myocardial adaptation to morphine are proposed, including sequential sympathetic and parasympathetic compensations. The American Society for Pharmacology and Experimental Therapeutics