Abstract
KEY POINTS: Distinct Ca(2+) channels work in a coordinated manner to grade Ca(2+) spark/spontaneous transient outward currents (STOCs) in rat cerebral arteries. The relative contribution of each Ca(2+) channel to Ca(2+) spark/STOC production depends upon their biophysical properties and the resting membrane potential of smooth muscle. Na(+) /Ca(2+) exchanger, but not TRP channels, can also facilitate STOC production. ABSTRACT: Ca(2+) sparks are generated in a voltage-dependent manner to initiate spontaneous transient outward currents (STOCs), events that moderate arterial constriction. In this study, we defined the mechanisms by which membrane depolarization increases Ca(2+) sparks and subsequent STOC production. Using perforated patch clamp electrophysiology and rat cerebral arterial myocytes, we monitored STOCs in the presence and absence of agents that modulate Ca(2+) entry. Beginning with Ca(V) 3.2 channel inhibition, Ni(2+) was shown to decrease STOC frequency in cells held at hyperpolarized (-40 mV) but not depolarized (-20 mV) voltages. In contrast, nifedipine, a Ca(V) 1.2 inhibitor, markedly suppressed STOC frequency at -20 mV but not -40 mV. These findings aligned with the voltage-dependent profiles of L- and T-type Ca(2+) channels. Furthermore, computational and experimental observations illustrated that Ca(2+) spark production is intimately tied to the activity of both conductances. Intriguingly, this study observed residual STOC production at depolarized voltages that was independent of Ca(V) 1.2 and Ca(V) 3.2. This residual component was insensitive to TRPV4 channel modulation and was abolished by Na(+) /Ca(2+) exchanger blockade. In summary, our work highlights that the voltage-dependent triggering of Ca(2+) sparks/STOCs is not tied to a single conductance but rather reflects an interplay among multiple Ca(2+) permeable pores with distinct electrophysiological properties. This integrated orchestration enables smooth muscle to grade Ca(2+) spark/STOC production and thus precisely tune negative electrical feedback.