Functional Adenylyl Cyclase Inhibition in Murine
Cardiomyocytes by 2'(3')-O-(N-Methylanthraniloyl)-Guanosine
5'-[γ-Thio]triphosphate

Abstract

β₁-Adrenergic receptor activation stimulates cardiac L-type
Ca²⁺ channels via adenylyl cyclases (ACs), with AC5 and AC6
being the most important cardiac isoforms. Recently, we have identified 2'(3')-O-(N-methylanthraniloyl)-guanosine 5'-[γ-thio-]triphosphate (MANT-GTPγS) as a potent competitive AC inhibitor. Intriguingly, MANT-GTPγS inhibits AC5 and -6 more potently than other cyclases. These data prompted us to study the effects of MANT-GTPγS on L-type Ca²⁺ currents (ICa,L) in ventricular myocytes of wild-type (WT) and AC5-deficient (AC5⁻/⁻) mice by whole-cell recordings. In wild-type myocytes, MANT-GTPγS attenuated ICa,L stimulation following isoproterenol
application in a concentration-dependent manner (control,
+77 ± 13%; 100 nM MANT-GTPγS, +43 ± 6%; 1 μM MANTGTPγS, +21 ± 9%; p ‹ 0.05). The leftward shift of currentvoltage
curves was abolished by 1 μM but not by 100 nM
MANT-GTPγS. In myocytes from AC5⁻/⁻ mice, the residual
stimulation of ICa,L was not further attenuated by the nucleotide, indicating AC5 to be the major AC isoform mediating acute β-adrenergic stimulation in WT mice. Interestingly, basal ICa,L was lowered by 1 M but not by 100 nM MANT-GTPγS. The decrease was less pronounced in myocytes from AC5⁻/⁻ mice compared with wild types (-23 ± 1 versus -40 ± 7%), indicating basal ICa,L to be partly driven by AC5. Collectively, we found a concentration-dependent inhibition of ICa,L by MANT-GTPγS, both under basal conditions and following β-adrenergic stimulation.
Comparison of data from wild-type and AC5-deficient mice
indicates that AC5 plays a major role in ICa,L activation and that MANT-GTPγS predominantly acts via AC5 inhibition.