Interplay among distinct Ca²⁺ conductances drives Ca²⁺ sparks/spontaneous transient outward currents in rat cerebral arteries

Abstract

Ca 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 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 entry. Beginning with Ca 3.2 channel inhibition, Ni was shown to decrease STOC frequency in cells held at hyperpolarized (-40 mV) but not depolarized (-20 mV) voltages. In contrast, nifedipine, a Ca 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 channels. Furthermore, computational and experimental observations illustrated that Ca 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 1.2 and Ca 3.2. This residual component was insensitive to TRPV4 channel modulation and was abolished by Na /Ca exchanger blockade. In summary, our work highlights that the voltage-dependent triggering of Ca sparks/STOCs is not tied to a single conductance but rather reflects an interplay among multiple Ca permeable pores with distinct electrophysiological properties. This integrated orchestration enables smooth muscle to grade Ca spark/STOC production and thus precisely tune negative electrical feedback.

Differential targeting and signalling of voltage‐gated T‐type Cav3.2 and L‐type Cav1.2 channels to ryanodine receptors in mesenteric arteries

“…T‐type channels in triggering Ca 2+ sparks depends on the membrane voltage of the cells (Hashad et al . ). Since L‐type channels are high voltage‐activated Ca 2+ channels, they cause Ca 2+ sparks at relatively high membrane potentials.…”

“…In contrast, T‐type channels are low voltage‐activated Ca 2+ channels, which can trigger Ca 2+ sparks only at relatively low membrane potentials (Hashad et al . ). Given that we mainly worked with isolated cells, it is possible that the higher contribution of L‐type channels could be due to a more depolarized state of the VSMCs.…”

“…Based on our results, we conclude that T-type channels seem to be responsible for 20-30% of the Ca 2+ spark events in mesenteric arteries, which are resistant to L-type Ca v 1.2 channel block. The relative contribution of L-type vs. T-type channels in triggering Ca 2+ sparks depends on the membrane voltage of the cells (Hashad et al 2017 ). B, same as A but after incubation of cells with methyl-β-cyclodextrin (dextrin, 10 mM, 90 min at room temperature).…”

“…potentials (Hashad et al 2017). Given that we mainly worked with isolated cells, it is possible that the higher contribution of L-type channels could be due to a more depolarized state of the VSMCs.…”

Voltage-dependent inward currents in smooth muscle cells of skeletal muscle arterioles

“…The voltage-dependence of sustained elevation of the bulk intracellular Ca 2+ due to L-type Ca 2+ channels was found to be from -40 to -15 mV, peaking at -30 mV [72]. In neurons, the voltage range for sustained activity of T-type Ca 2+ channels is from -90 to -30 mV [73]. Although we found that T-type Ca 2+ channels in smooth muscle cells of skeletal muscle arterioles activate at least 20 mV more positively than in neurons, their activation occurs at more negative voltages in comparison with L-type Ca 2+ channels (S2 Fig). Relevant to the voltage-dependence of activation of T-type Ca 2+ channels, we found that the resting membrane potential in our experimental conditions was about -70 mV (Fig 1).…”