Calcium sparklets regulate local and global calcium in murine arterial smooth muscle

In arterial smooth muscle, protein kinase Cα (PKCα) coerces discrete clusters of L-type Ca²⁺ channels to operate in a high open probability mode, resulting in subcellular domains of nearly continual Ca²⁺ influx called 'persistent Ca²⁺ sparklets'. Our previous work suggested that steady-state Ca²⁺ entry into arterial myocytes, and thus global [Ca²⁺]_i, is regulated by Ca²⁺ influx through clusters of L-type Ca²⁺ channels operating in this persistently active mode in addition to openings of solitary channels functioning in a low-activity mode. Here, we provide the first direct evidence supporting this 'Ca²⁺ sparklet' model of Ca²⁺ influx at a physiological membrane potential and external Ca²⁺ concentration. In support of this model, we found that persistent Ca²⁺ sparklets produced local and global elevations in [Ca²⁺]_i. Membrane depolarization increased Ca²⁺ influx via low-activity and high-activity persistent Ca²⁺ sparklets. Our data indicate that Ca²⁺ entering arterial smooth muscle through persistent Ca²⁺ sparklets accounts for approximately 50% of the total dihydropyridine-sensitive (i.e. L-type Ca²⁺ channel) Ca²⁺ influx at a physiologically relevant membrane potential (-40 mV) and external Ca²⁺ concentration (2 mM). Consistent with this, inhibition of basal PKCα-dependent persistent Ca²⁺ sparklets decreased [Ca²⁺]_i by about 50% in isolated arterial myocytes and intact pressurized arteries. Taken together, these data support the conclusion that in arterial smooth muscle steady-state Ca²⁺ entry and global [Ca²⁺]_i are regulated by low-activity and PKCα-dependent high-activity persistent Ca²⁺ sparklets.