Steffen‐Sebastian Bolz

We hypothesized that nitric oxide (NO) and the endothelium‐dependent hyperpolarizing factor (EDHF) may dilate microvessels by different cellular mechanisms, namely Ca²⁺‐desensitization versus decrease in intracellular free calcium. Effects of acetylcholine (ACh) and the NO donors sodium nitroprusside (SNP, 0.1–10 μmol l⁻¹) and S‐Nitroso‐N‐acetyl‐D,L‐penicillamine (SNAP, 0.01–10 μmol l⁻¹) on intracellular calcium ([Ca²⁺]_i, fura 2) and vascular diameter (videomicroscopy) were studied in isolated resistance arteries from hamster gracilis muscle (194±12 μm) pretreated with indomethacin and norepinephrine. Membrane potential changes were determined using 1,3‐dibutylbarbituric acid trimethineoxonol (DiBAC₄(3)). ACh (0.1 and 1 μmol l⁻¹)‐induced dilations were associated with a [Ca²⁺]_i decrease (by 13±3 and 32±4%) and hyperpolarization of vascular smooth muscle (VSM, by 12±1% at 1 μmol l⁻¹ ACh). N^ω‐nitro‐L‐arginine (L‐NA, 30 μmol l⁻¹) partially inhibited the dilation but did not affect VSM [Ca²⁺]_i decreases or hyperpolarization. In contrast, the K_Ca channel inhibitors tetrabutylammonium (TBA, 1 mmol l⁻¹) and charybdotoxin (ChTX, 1 μmol l⁻¹) abolished the ACh‐induced [Ca²⁺]_i decrease and the hyperpolarization in VSM while a significant dilation remained (25 and 40%). This remaining dilation was abolished by L‐NA. ChTX did not affect [Ca²⁺]_i increase and hyperpolarization in endothelial cells. SNP‐ or SNAP‐induced dilations were not associated with decreases in VSM [Ca²⁺]_i or hyperpolarization although minor transient decreases in VSM [Ca²⁺]_i were observed at high concentrations. These data suggest that ACh‐induced dilations in microvessels are predominantly mediated by a factor different from NO and PGI₂, presumably EDHF. EDHF exerts dilation by activation of K_Ca channels and a subsequent decrease in VSM [Ca²⁺]_i, NO dilates the microvessels in a calcium‐independent manner. British Journal of Pharmacology (1999) 128, 124–134; doi:10.1038/sj.bjp.0702775