It has been generally accepted that, of the five subtypes of muscarinic receptors (M-1-M-5), only the M-2 subtype is expressed in mammalian heart. This notion has recently been challenged by a series of reports indicating that mRNAs for some or all non-M-2 subtypes are also present in mammalian heart, in parallel with the M-2 mRNA. However, the quantities of relevant mRNAs reported to be present in the heart are not known, which makes it difficult to evaluate their likely significance. We measured the concentrations of the five muscarinic mRNAs by competitive reverse transcription-polymerase chain reaction and discovered that the M-2 mRNA represents more than 90% of total muscarinic mRNAs in rat atria and in either ventricle. The concentrations of total muscarinic mRNAs and of the M-2 mRNA were more than twice as high in the atria than in the ventricles. mRNAs for all non-M-2 muscarinic receptor sub-types were also detected but represented less than 1% (M-1 and M-4), less than 3% (M-3), and less than 5% (M-5) of total muscarinic RNAs in the atria and ventricles. The findings support the concept of the prevalent role of the M-2 muscarinic receptors in the cholinergic control of the heart. When the same method of quantitation was applied to rat cerebral cortex, mRNAs for individual subtypes were found to represent 36% (M-1), 21% (M-2), 25% (M-3), 11% (M-4), and 7% (M-5) of total muscarinic mRNAs.
