Potential role of endothelin and nitric oxide in physiology and pathophysiology of the lower urinary tract

Inhibition of vascular smooth muscle inward-rectifier K+ channels restores myogenic tone in mouse urinary bladder arterioles

Prolonged decreases in urinary bladder blood flow are linked to overactive and underactive bladder pathologies. However, the mechanisms regulating bladder vascular reactivity are largely unknown. To investigate these mechanisms, we examined myogenic and vasoactive properties of mouse bladder feed arterioles (BFAs). Unlike similar-sized arterioles from other vascular beds, BFAs failed to constrict in response to increases in intraluminal pressure (5-80 mmHg). Consistent with this lack of myogenic tone, arteriolar smooth muscle cell membrane potential was hyperpolarized (-72.8 ± 1.4 mV) at 20 mmHg and unaffected by increasing pressure to 80 mmHg (-74.3 ± 2.2 mV). In contrast, BFAs constricted to the thromboxane analog U-46619 (100 nM), the adrenergic agonist phenylephrine (10 µM), and KCl (60 mM). Inhibition of nitric oxide synthase or intermediate- and small-conductance Ca²⁺-activated K⁺ channels did not alter arteriolar diameter, indicating that the dilated state of BFAs is not attributable to overactive endothelium-dependent dilatory influences. Myocytes isolated from BFAs exhibited BaCl₂ (100 µM)-sensitive K⁺ currents consistent with strong inward-rectifier K⁺ (K_IR) channels. Notably, block of these K_IR channels "restored" pressure-induced constriction and membrane depolarization. This suggests that these channels, in part, account for hyperpolarization and associated absence of tone in BFAs. Furthermore, smooth muscle-specific knockout of K_IR2.1 caused significant myogenic tone to develop at physiological pressures. This suggests that 1) the regulation of vascular tone in the bladder is independent of pressure, insofar as pressure-induced depolarizing conductances cannot overcome K_IR2.1-mediated hyperpolarization; and 2) maintenance of bladder blood flow during bladder filling is likely controlled by neurohumoral influences.

Carotid atherosclerosis in type 2 diabetes mellitus: potential role of endothelin-1, lipoperoxides, and prostacyclin

Factors were studied that may initiate macroangiopathy or enhance or aggravate its pathogenesis in patients with type 2 diabetes mellitus. A total of 151 diabetics were compared with healthy controls (n=50); all patients and subjects were normotensive and without renal failure. Plasma endothelin-1 and free radical levels were measured. In addition, plasma prostacyclin levels were assessed by assaying its stable, spontaneous, breakdown product 6-keto-prostaglandin-F1a. Diabetics were divided into three groups: those with clinically evident macroangiopathy and those with early or without atherosclerosis (as determined by the carotid intima-media thickness. Plasma endothelin-1 levels were increased in all diabetics with atherosclerosis. Plasma free radical levels were increased in diabetics with macroangiopathy when compared with control subjects. The plasma levels of 6-keto-prostaglandin-F1a were slightly, but significantly, decreased in the diabetics with macroangiopathy when compared with control subjects. The carotid intima-media thickness was significantly greater in diabetics without macroangiopathy when compared with the controls. Furthermore, the intima-media thickness increased significantly in this group of diabetics but not in the controls over a 30-month follow-up period. Several factors may contribute to atherogenesis in diabetics. These include increased plasma endothelin-1 and free radical levels as well as a deficiency of prostacyclin. These factors may become targets for intervention as well as markers of disease progression.