A 61603 hydrobromide (CAS 107756-30-9) is a potent and highly selective agonist of the 1A-adrenergic receptor one of the 1-adrenergic receptor subtypes involved in cellular growth and proliferation Compared to its activity at 1B- and 1D-adrenergic receptors A 61603 demonstrates approximately 35-fold higher potency for 1A-AR In cellular assays it robustly induces phosphoinositide hydrolysis in 1A-AR-expressing fibroblasts (pEC50 8 24) and in neonatal rat cardiomyocytes it increases calcium transient frequency in a dose-dependent manner (EC50 6 9 nM) In vascular models A 61603 markedly elevates perfusion pressure demonstrating significantly greater efficacy than noradrenaline This compound serves as a valuable tool for delineating 1A-AR-mediated signaling and vascular regulation