Mammalian heart has been reported to express AC isozymes (types V and VI) that are inhibited by < microM [Ca2+]; avian heart has been reported to express adenylyl cyclase activity that is inhibited by < microM [Ca2]. We have used reverse transcription polymerase chain reaction (RT-PCR) to determine that type V and VI AC mRNAs are present in freshly isolated ventricular myocytes. Subsequent RNase protection assays revealed that that the type V signal is 4-5 times that for the type VI isozyme. In situ hybridization with high specific activity cRNA probes combined with immunocytochemistry with a chick anti-myosin antibody was used to probe the cellular origins of type V and type VI AC signals. These studies show that myocytes contain messages for both the type V and VI isozymes but that AC V is the major isoform. Interestingly, while the type V AC mRNA appears to be localized primarily, if not exclusively, in myocytes, the signal for type AC VI mRNA in non-myocytes is stronger than in myocytes.