A 61603 is a synthetic small molecule and potent agonist of the G protein-coupled alpha 1A receptor (α1A), at least 35-fold selective vs α1B or α1D. More potent stimulant of phosphoinositide hydrolysis than norepinephrine, and inhibited by prazosin. Key role in modulation of the sympathetic nervous system, and increases spontaneous calcium transients more potently than phenylephrine. May be implicated in diseases such as hypertension.