Background--Neuregulins are required for maintenance of acetylcholine receptor-inducing activity of nicotinic receptors in neurons and skeletal muscle, but effects of neuregulins on muscarinic receptors are not known. In the normal heart, parasympathetic activation counterbalances -adrenergic activation. To test the hypothesis that neuregulins modify parasympathetic function in the heart, we studied cardiomyocytes from mice heterozygous for neuregulin-1 gene deletion (NRG-1^+/-) and examined the effects of -adrenergic stimulation on contractility in the presence and absence of the muscarinic agonist carbachol.

Methods and Results--We evaluated contraction and intracellular Ca²⁺ transients ([Ca²⁺]_i) in left ventricular (LV) myocytes loaded with Fluo-3 from NRG-1^+/- and wild-type (WT) mice. Under baseline conditions (0.5 Hz, 1.5 mmol/L [Ca²⁺]_o, 25°C), characteristics of myocyte contraction/relengthening and systolic/diastolic [Ca²⁺]_i were not different between WT and NRG-1^+/- mice. The steady-state increases in fractional shortening (FS) and peak-systolic [Ca²⁺]_i in response to isoproterenol were similar in both groups. In WT myocytes stimulated with isoproterenol, carbachol decreased FS, peak-systolic [Ca²⁺]_i, and cAMP levels. In NRG-1^+/- myocytes, carbachol did not attenuate either FS or peak-systolic [Ca²⁺]_i, associated with the failure to decrease cAMP levels. Investigation of muscarinic receptor signaling showed no difference of LV protein levels of muscarinic M₂ receptors or G protein G_i1,2, G_i3, and G_o subunits.

Conclusions--Cardiomyocytes deficient in neuregulin signaling are unable to adequately counterbalance -adrenergic activation by inhibitory parasympathetic activity. This mechanism may contribute to the known increased risk of heart failure in injured human hearts when neuregulin signaling is suppressed.