SMAKing Ca2+ sparks in arterial myocytes.

In the current issue of The Journal of Physiology, Essin et al. (2007) examined the relationship between Cav1.2 L-type Ca2+ channel function and Ca2+ spark activity in arterial myocytes. To do this, they took advantage of the SMAKO mouse (Moosmang et al. 2003) – a conditional, vascular smooth muscle-specific Cav1.2 knock-out mouse. Using SMAKO mice they provide what represents the strongest, most direct demonstration that, in arterial myocytes, local Ca2+ signalling between Cav1.2 channels and ryanodine receptors (RyRs) is not required for the activation of Ca2+ sparks. Rather, Cav1.2 activity influences Ca2+ spark activity in a subtler, indirect way by controlling sarcoplasmic reticulum (SR) Ca2+ load. The functional implications of these findings are discussed below. Cav1.2 is the predominant L-type Ca2+ channel isoform expressed in murine arterial smooth muscle (Moosmang et al. 2003; Navedo et al. 2007; Zhang et al. 2007). Local [Ca2+]i signals (called ‘Ca2+ sparklets’) resulting from the opening of individual L-type Ca2+ channels have been imaged in ventricular (Wang et al. 2001) and arterial myocytes (Navedo et al. 2005). Interestingly, Ca2+ sparklet activity varies within the sarcolemma of arterial myocytes (Navedo et al. 2005, 2006). The majority of L-type Ca2+ channels in these cells operate in low opening probability mode that causes small, sporadic elevations in [Ca2+]i (i.e. low activity Ca2+ sparklets). In contrast, discrete clusters of L-type Ca2+ channels associated with PKCα operate in a sustained, high activity mode that results in substantial Ca2+ influx (i.e. persistent Ca2+ sparklets) (Navedo et al. 2005, 2006; Amberg et al. 2007). On the basis of these data, a model for steady-state Ca2+ influx in arterial myocytes has been proposed in which membrane depolarization increases Ca2+ influx by activation of low and high activity persistent Ca2+ sparklets. Like Ca2+ sparklets, Ca2+ sparks increase [Ca2+]i locally near the surface sarcolemma (Nelson et al. 1995). However, unlike Ca2+ sparklets, Ca2+ sparks do not directly cause global increases in [Ca2+]i and contraction. Instead, Ca2+ sparks activate closely apposed, large-conductance, Ca2+-activated K+ (BK) channels, which hyperpolarize arterial smooth muscle and consequently decrease L-type Ca2+ channel activity, arterial wall [Ca2+]i, and myogenic tone (Nelson et al. 1995). Thus, dynamic regulation of BK channel activity by Ca2+ sparks is critical for the regulation of arterial function. In ventricular myocytes, Ca2+ sparks are rapidly activated by Ca2+ influx through L-type Ca2+ channels (presumably Cav1.2) by the mechanism of Ca2+-induced Ca2+ release (Lopez-Lopez et al. 1995; Wang et al. 2001). Activation of Ca2+ sparks by L-type Ca2+ channels occurs in specialized regions where the SR (i.e. junctional SR) comes into close apposition to the sarcolemma of ventricular myocytes. During an action potential, the simultaneous activation of multiple L-type Ca2+ channels results in the local recruitment of numerous Ca2+ sparks in these cells. Tight coupling between Cav1.2 L-type Ca2+ channels and Ca2+ sparks is a fundamental tenet of the local control model of excitation–contraction coupling in heart. Like ventricular myocytes, smooth muscle cells express Cav1.2 channels and have junctional SR, where Ca2+ sparks occur. However, as Essin et al. (2007) elegantly demonstrated, tight, cardiac-like Cav1.2-RyR coupling is not a major regulator of Ca2+ spark activity in arterial myocytes. The data supporting this view are compelling. (1) The frequency and amplitude of Ca2+ sparks and BK currents is lower in SMAKO than in wild-type arterial myocytes. (2) Dihydropyridine agonists and antagonists do not rapidly increase or eliminate BK currents and Ca2+ sparks. (3) Global [Ca2+]i and SR Ca2+ load are lower in SMAKO than in wild-type arterial myocytes. (4) Increasing [Ca2+]i increased BK currents (presumably by activating Ca2+ sparks) in SMAKO and wild-type myocytes. (5) Unlike ventricular myocytes (Lopez-Lopez et al. 1995; Wang et al. 2001), first-latency histograms of Ca2+ sparks showed only a weak voltage dependence. (6) The average latency of Ca2+ sparks between –30 and +50 mV occurred at > 100 ms, a value that is much longer than what would be expected for Cav1.2 channels. If, in arterial myocytes, Ca2+ sparks were locally controlled by Ca2+ influx via L-type Ca2+ channels, Ca2+ sparks would have kinetics resembling those of L-type Ca2+ channels. A limitation in the Essin et al. (2007) study is that they did not simultaneously record Ca2+ sparklet and spark activity. This is important because, as noted above, L-type Ca2+ channel activity varies within the sarcolemma of arterial myocytes. Is Ca2+ spark activity higher near persistent Ca2+ sparklet sites? Is SR Ca2+ locally controlled by the level of nearby Ca2+ sparklet activity? And, is the coupling strength between Cav1.2 channels and Ca2+ sparks modulated by vasodilators and/or vasoconstrictors? Future studies should address these important questions. Doing so will reveal in greater detail the inner workings of arterial smooth muscle. The challenge will be to integrate these studies on local excitation–contraction coupling into a coherent, integrative model of arterial function.