6‐Nitrodopamine Potentiates Catecholamine‐Induced Ca ²⁺ _i Release in Human Aortic Smooth Muscle Cells and Modulates Vascular Smooth Muscle Contractility

ABSTRACT Background In vascular smooth muscle, rises in intracellular calcium [Ca 2+ ] i drive contraction downstream of α 1 ‐adrenoceptor activation via IP3. Endothelium‐derived 6‐nitrodopamine (6‐ND) augments cardiac catecholamine actions; however, its effect on [Ca 2+ ] i is unknown. We hypothesized that 6‐ND would increase intracellular [Ca 2+ ] i and potentiate catecholamine‐induced [Ca 2+ ] i signalling in vascular smooth muscle cells, with resultant functional effects on vascular tone when measured in vitro. Methods Human aortic smooth muscle cells (HASMCs) were loaded with fura‐2 AM (1 μM) and [Ca 2+ ] i measured using a CLARIOstar plate reader after addition of Hanks’ balanced salt solution. Rat thoracic aorta rings, with the endothelium removed, were mounted in Krebs–Henseleit baths, and isometric force was recorded via PowerLab. Results 6‐ND and classical catecholamines evoked concentration‐dependent increases in HASMC Ca 2+ flux, with 6‐ND displaying the greatest potency. 6‐ND potentiated the increases in HASMC Ca 2+ flux induced by the classical catecholamines. Tetrodotoxin (TTX) caused a concentration‐dependent inhibition of responses to 6‐ND and dopamine but did not alter noradrenaline‐ or adrenaline‐induced [Ca 2+ ] i rise. In endothelium‐denuded aortic rings, 6‐ND potentiated contractions elicited by catecholamines, and this potentiation was abolished by TTX. Conclusions 6‐ND is a potent modulator of [Ca 2+ ] i in HASMCs and enhances catecholamine‐driven vasoconstriction. Both effects are blocked by TTX, indicating that 6‐ND modulates voltage‐gated sodium channels upstream of Ca 2+ entry/release in vascular smooth muscle cells.