Rationale

Gq signaling in cardiac myocytes is classically considered toxic. Targeting Gq directly to test this is problematic, because cardiac myocytes have many Gq-coupled receptors.

Objective

Test whether Gq coupling is required for the cardioprotective effects of an alpha-1A-AR (adrenergic receptor) agonist.

Methods and results

In recombinant cells, a mouse alpha-1A-AR with a 6-residue substitution in the third intracellular loop does not couple to Gq signaling. Here we studied a knockin mouse with this alpha-1A-AR mutation. Heart alpha-1A receptor levels and antagonist affinity in the knockin were identical to wild-type. In wild-type cardiac myocytes, the selective alpha-1A agonist A61603-stimulated phosphoinositide-phospholipase C and myocyte contraction. In myocytes with the alpha-1A knockin, both A61603 effects were absent, indicating that Gq coupling was absent. Surprisingly, A61603 activation of cardioprotective ERK (extracellular signal-regulated kinase) was markedly impaired in the KI mutant myocytes, and A61603 did not protect mutant myocytes from doxorubicin toxicity in vitro. Similarly, mice with the α1A KI mutation had increased mortality after transverse aortic constriction, and A61603 did not rescue cardiac function in mice with the Gq coupling-defective alpha-1A receptor.

Conclusions

Gq coupling is required for cardioprotection by an alpha-1A-AR agonist. Gq signaling can be adaptive.