We examined the possibility of functional and molecular expression of volume‐regulated Cl− channels in vascular smooth muscle using the whole‐cell patch‐clamp technique and quantitative reverse transcriptase‐polymerase chain reaction (RT‐PCR) on cells from canine pulmonary and renal arteries. Decreasing external osmolarity induced cell swelling, which was accompanied by activation of Cl−‐dependent outward‐rectifying membrane currents with an anion permeability sequence of SCN− > I− > Br− > Cl− > aspartate−. These currents were sensitive to block by DIDS, extracellular ATP and the antioestrogen compound tamoxifen. Experiments were performed to determine whether the molecular form of the volume‐regulated chloride channel (ClC‐3) is expressed in pulmonary and renal arteries. Quantitative RT‐PCR confirmed expression of ClC‐3 in both types of smooth muscle. ClC‐3 expression was 76.4 % of β‐actin in renal artery and 48.0 % of β‐actin in pulmonary artery. We conclude that volume‐regulated Cl− channels are expressed in vascular smooth muscle cells and exhibit functional properties similar to those found in other types of cells, presumably contributing to the regulation of cell volume, electrical activity and, possibly, myogenic tone.

1 The role of nitric oxide (NO) in the regulation of uterine contractility has yet to be clearly defined. We evaluated the effect of NO (in the form of S-nitroso-L-cysteine, CysNO) upon uterine contractility and guanosine 3',5'-cyclic monophosphate (cyclic GMP) accumulation in pregnant and nonpregnant guinea-pig myometrium.2 While CysNO had no effect upon spontaneous contractile activity in either pregnant or nonpregnant uterine tissues, addition of CysNO resulted in an immediate and reversible relaxation of oxytocin-or acetylcholine (ACh)-evoked contractions. 3 Relaxation of agonist-evoked contractions in response to CysNO was associated with significant elevations in intracellular cyclic GMP concentrations ([cyclic GMP]i). 4 Elevations in [cyclic GMP]j were not required for relaxation, as inhibition of guanylyl cyclase by methylene blue prevented [cyclic GMP]j accumulation while having no effect upon the ability of CysNO to relax agonist-evoked contractions. 5 Addition of the cyclic GMP-analogues, 8-Br-cyclic GMP and PET-cyclic GMP, only at high concentrations, produced partial relaxation of agonist-contracted tissues, suggesting the possibility that cyclic GMP may be sufficient but not necessary for myometrial relaxation. 6 Our studies not only provide evidence for a functional role for NO-modulation of agonist-evoked contractions in the pregnant and nonpregnant guinea-pig uterus, but also that these occur by a mechanism which is not dependent upon guanylyl cyclase activity.

1 We evaluated the e ect of the nitric oxide (NO) donor CysNO (S-nitroso-L-cysteine) and endogenous NO upon spontaneous contractility in non-pregnant cynomolgus monkeys. We also assessed the role of intracellular guanosine 3',5'-cyclic monophosphate ([cyclic GMP] i ) as a second messenger for NO in monkey uterine smooth muscle. 2 CysNO reduced spontaneous contractility by 84% (P50.05) at maximal concentrations, and signi®cantly elevated [cyclic GMP] i (P50.05). However, increases in [cyclic GMP] i were not required for CysNO-induced relaxations; CysNO inhibited contractile activity despite the complete inhibition of guanylyl cyclase by methylene blue or LY83,583. 3 Analogues of cyclic GMP had no signi®cant e ect upon spontaneous contractile activity. L-arginine produced a 62% reduction in spontaneous activity (P50.05) while D-arginine had no e ect. The competitive nitric oxide synthase (NOS) inhibitor N o -nitro-L-arginine (L-NOARG) not only blocked Larginine-induced relaxations, but also signi®cantly increased spontaneous contractile activity when added alone (P50.05); the inactive D-enantiomer of NOARG had no such e ect. 4 While both endogenous NO and the NO donor CysNO relax monkey myometrium, this e ect is not causally related to CysNO-induced elevations in [cyclic GMP] i . The failure of cyclic GMP analogues to alter monkey uterine smooth muscle tension also argues against a role for [cyclic GMP] i in the regulation of uterine contractility. Not only do these ®ndings argue for the existence of a functionally-relevant NOS in the monkey uterus, but increases in contractile activity seen in the presence of NOS inhibitors suggest a role for NO in the moment-to-moment regulation of contractile activity in this organ.

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