6-Nitrodopamine Potentiates Catecholamine-Induced Ca²⁺ _i Release in Human Aortic Smooth Muscle and Modulates Vascular...

In vascular smooth muscle, rises in intracellular calcium [Ca²⁺]_i drive contraction downstream of α₁-adrenoceptor activation via IP3. Endothelium-derived 6-nitrodopamine (6-ND) augments cardiac catecholamine actions; however, its effect on [Ca²⁺]_i is unknown. We hypothesized that 6-ND would increase intracellular [Ca²⁺]_i and potentiate catecholamine-induced [Ca²⁺]_i signalling in vascular smooth muscle cells, with resultant functional effects on vascular tone when measured in vitro.

Human aortic smooth muscle cells (HASMCs) were loaded with fura-2 AM (1 μM) and [Ca²⁺]_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.

6-ND and classical catecholamines evoked concentration-dependent increases in HASMC Ca²⁺ flux, with 6-ND displaying the greatest potency. 6-ND potentiated the increases in HASMC Ca²⁺ 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 [Ca²⁺]_i rise. In endothelium-denuded aortic rings, 6-ND potentiated contractions elicited by catecholamines, and this potentiation was abolished by TTX.

6-ND is a potent modulator of [Ca²⁺]_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 Ca²⁺ entry/release in vascular smooth muscle cells.