Phenylephrine acts via IP3-dependent intracellular NO release to stimulate L-type Ca2+ current in cat atrial myocytes.

α1-Adrenergic receptor (α1-AR) stimulation plays important roles in the regulation of cardiac contraction, cell growth, hypertrophy and cardioprotection (Li et al. 1997). Classically, α1-ARs are coupled via pertussis toxin (PTX)-insensitive G-proteins (Gq), although coupling to Gi-protein has been reported (Steinberg et al. 1985; Han et al. 1989; Keung & Karliner, 1990; Perez et al. 1993). In general, α1-AR stimulation activates phospholipase C (PLC) to hydrolyse phosphatidylinositol 4,5-bisphosphate (PIP2), leading to the production of diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). DAG activates protein kinase C (PKC) and in cardiac muscle IP3 enhances intracellular Ca2+ release (Nosek et al. 1986; Fabiato, 1986; Zima & Blatter, 2004). Atrial myocytes express functional IP3 receptors (IP3R type-2) at 6–10 times higher levels than in ventricular myocytes and IP3Rs colocalize with ryanodine receptors in the subsarcolemmal space (Lipp et al. 2000; Mackenzie et al. 2002). In permeabilized cat atrial myocytes, exposure to IP3 stimulates local Ca2+ release, i.e. Ca2+ sparks, from the sarcoplasmic reticulum (SR) (Zima & Blatter, 2004). Although IP3 signalling has been implicated in atrial excitation–contraction coupling and/or atrial arrhythmias (Lipp et al. 2000; Mackenzie et al. 2002; Zima & Blatter, 2004), the functional role of IP3 signalling in atrial muscle is still not clear. In general, phenylephrine (PE), an α1-AR agonist, exerts positive inotropic effects in adult ventricular muscle (Hartmann et al. 1988; Hescheler et al. 1988; Ertl et al. 1991; Terzic et al. 1992) as well as in rat (Ertl et al. 1991; Jahnel et al. 1994) and human (Schumann et al. 1978; Skomedal et al. 1985; Jahnel et al. 1992a) atrial muscle. The majority of studies indicate that the positive inotropic effect of α1-AR stimulation is not primarily mediated via increases in ICa,L (Ertl et al. 1991). However, in guinea pig ventricular myocytes PE acts via PKC signalling to stimulate ICa,L and contractile amplitude (Woo & Lee, 1999). Also, in rat ventricular myocytes PE can increase ICa,L, possibly via PKC activation when the intracellular environment is not disturbed by intracellular dialysis (Zhang et al. 1998). In neonatal rat ventricular myocytes (Liu et al. 1994), PE increases ICa,L (Liu et al. 1994) and chronic exposure to PE in culture induces hypertrophy and increases ICa,L (Gaughan et al. 1998). PE may increase myofilament Ca2+ sensitivity (Terzic et al. 1992) possibly via PKC activation. In rat atria, PE increases intracellular Ca2+ uptake via cAMP-dependent stimulation of voltage-dependent Ca2+ channels (Jahnel et al. 1994), and possibly in part via secondary changes in Na+/Ca2+ exchange (Jahnel et al. 1991, 1992b, 1994). In ventricular muscle, the positive inotropic effect of α1-AR stimulation does not correlate with increases in cellular cAMP levels (Schumann et al. 1975; Brodde et al. 1978; Bogoyevitch et al. 1993). Clearly, the cellular mechanisms underlying α1-AR stimulation in heart are diverse and not entirely understood. In cat atrial myocytes, stimulation of β2-ARs (Dedkova et al. 2002) or muscarinic receptors (Wang et al. 1998; Dedkova et al. 2003) stimulates Gi-mediated release of intracellular NO (NOi). In human (Kirstein et al. 1995) and cat (Wang et al. 2002) atrial myocytes, NO stimulates ICa,L via cGMP-induced inhibition of phosphodiesterase (PDE) type III activity to enhance endogenous cAMP-dependent PKA activity. The purpose of the present study was to determine whether α1-AR stimulation regulates ICa,L in cat atrial myocytes and if so, whether NOi signalling plays a role. The results indicate that α1-ARs act via PTX-insensitive G-proteins to increase ICa,L via NOi signalling. Moreover, α1-AR stimulation activates NOi release via PI-3K/Akt and IP3-dependent signalling mechanisms. IP3-dependent Ca2+ signalling may enhance local subsarcolemmal SR Ca2+ release required to activate Ca2+/calmodulin (CaM)-dependent eNOS. A portion of this work has been presented in abstract form (Lipsius et al. 2003).