Figure 6. Model for Ca²⁺ movements during PE-induced [Ca²⁺]_i oscillations in VSMCs from rabbit IVC.

IP₃ generated as a consequence of α-adrenergic activation releases Ca²⁺ towards the myoplasm to activate the myofilaments. Subsequent Ca²⁺ removal occurs partially through the PMCA, while the remainder is taken up into the SR by the SERCA. ROCs/SOCs, activated by receptor activation and SR depletion, allow entry of mainly Na⁺ and some Ca²⁺. This Na⁺ entry raises [Na⁺]_i in a restricted space between the plasma membrane and the SR membrane and drives NCX in the reverse mode to supply Ca²⁺ to SERCA to refill the SR lumen, thereby completing the cycle. PE, phenylephrine; NCX, sodium/calcium exchanger; ROC/SOC, receptor-operated channel/store-operated channel; SERCA, sarcoplasmic- endoplasmic reticulum Ca²⁺-ATPase; L-type VGCC, L-type voltage-gated Ca²⁺ channel; PMCA, plasma membrane Ca²⁺-ATPase; IP₃R, IP₃-sensitive SR Ca²⁺ release channel; SR, sarcoplasmic reticulum.