Regulation of basal cAMP levels in rabbit cardiomyocytes

Abstract

We recently found that basal cAMP levels critically influence the timecourse of agonist‐induced cAMP signals. We sought to identify which if any phosphodiesterases (PDEs) control basal cAMP levels in isolated rabbit cardiomyocytes. cAMP concentration near the plasma membrane of cardiomyocytes was monitored by measuring current through adenovirus‐expressed cyclic nucleotide‐gated (CNG) channels. In the absence of receptor agonists basal cAMP levels were low. A PDE inhibitor cocktail (PDE types 1, 3, and 4) triggered transient increases in cAMP and subsequent addition of a non‐selective PDE inhibitor caused no further increase. These data revealed that adenylyl cyclase (AC) and PDE are constitutively active in rabbit cardiomyocytes and that basal cAMP levels are primarily controlled by 3 PDE types. Any one of the 3 type‐specific PDE inhibitors caused small rises in cAMP. Addition of any second inhibitor triggered another rise, suggesting that the 3 PDE types work in concert to maintain basal cAMP levels. 1 μM H89 (a PKA inhibitor) reduced the rate of cAMP decline in the presence of PDE inhibitors, indicating that PKA‐mediated feedback contributed to the transient nature of these signals. In conclusion, basal cAMP is highly regulated by a balance between AC and PDE activity and increases in cAMP trigger PKA‐mediated feedback that returns cAMP to its basal level. Supported by HL74278, HL20648.