Effects of the potassium channel openers KRN4884 and levcromakalim on the contraction of rat aorta induced by A23187, compared with nifedipine

Contractile responses of the human umbilical artery to KCl and serotonin in Ca-free medium and the effects of levcromakalim

It is known that K(ATP) channel openers inhibit the release and refilling of Ca(2+) from intracellular stores. The present study was designed to test the effects of levcromakalim in human umbilical artery (HUA) rings stimulated by serotonin (5-HT) and KCl in Ca-free medium. Umbilical cords were obtained at vaginal or cesarean deliveries from healthy, term pregnancies. After the isolation, HUA rings were placed in organ baths in solution with indomethacin (10(-5) M) and N(G)-nitro-L-arginine methyl ester (L-NAME) (10(-3) M) at 37 degrees C and aerated with 95% O(2) and 5% CO(2) for the measurement of isometric force. In Ca-free solution with Ethylene glycol-bis (ss-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) (2 mM) the contractions produced by 5-HT (10(-6) M) and KCl (40 mM) decreased significantly. Afterwards, HUA rings were treated with 5-HT and KCl in repeated manner in Ca-free medium. In contrast to KCl, 5-HT induced contractions reduced in each application, progressively. Levcromakalim (10(-4) M) abolished the contractions elicited by 5-HT. On the other hand, levcromakalim had a little but significant inhibitory effect on KCl induced contraction in Ca-free medium. These results suggest that Ca(2+) is not the only transduction pathway in KCl produced contractions of HUA smooth muscle cells.

Effects of KRN4884, a Novel K+ Channel Opener, on Ionic Currents in Rabbit Femoral Arterial Myocytes

Effects of KRN4884 (5-amino-N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine), a novel K(+) channel opener, on ionic currents were examined in rabbit femoral arterial myocytes (RFAMs). Under whole-cell clamp conditions where cells were superfused with 5.9 mM K(+) bathing solution, KRN4884 elicited an outward current at -30 mV. KRN4884-induced current had a reversal potential of -78 mV and was abolished by application of glibenclamide (glib). KRN4884 was approximately 43 times more potent than levcromakalim in activating an ATP-sensitive K(+) current (I(K-ATP)). On the other hand, KRN4884 affected neither voltage-dependent Ca(2+) nor delayed rectifier K(+) channel currents. In the inside-out patch clamp configuration where cells were superfused with the symmetrical 140 mM K(+) solution, KRN4884 activated 47 pS K(+) channels in the presence of adenosine diphosphate. Similar 47 pS K(+) channels, which were reversibly inhibited by glib, were recorded under outside-out patch conditions. Using RT-PCR analysis, we found that inward rectifier K channel 6.1 (Kir6.1) and sulfonylurea 2B (SUR2B) transcripts were predominantly expressed in rabbit femoral artery. These results indicate that KRN4884 potently activates I(K-ATP) in RFAMs. The KRN4884-sensitive 47 pS K(+) channel activity underlying I(K-ATP) is a vascular type K(ATP) channel consisting of Kir6.1 and SUR2B and has similar characteristics to those of ATP-sensitive K(+) channels activated by K(+) channel openers in other types of smooth muscles.

The vaso-contractile action of zooxanthellatoxin-B from a marine dinoflagellate is mediated via Ca2+influx in the rabbit aorta

We previously showed that zooxanthellatoxin-B, isolated from dinoflagellate, caused a sustained contraction of the aorta in an external Ca2+-dependent manner. To clarify the role of Ca2+in this action, we examined the effects of zooxanthellatoxin-B as well as a depolarizing stimulus (60 mM KCl), using the simultaneous recording for cytosolic Ca2+level (fura-2) and developed tension in the rabbit aorta. KCl (60 mM) elicited a rapid cytosolic Ca2+elevation followed by a pronounced contraction, and time required for half-maximum contraction was 2 min. Zooxanthellatoxin-B caused an increase in cytosolic Ca2+followed by a gradual contraction, with a time for half-maximum contraction of 5–10 min in a concentration-dependent manner. We found a strong correlation between Ca2+elevation and the contraction in zooxanthellatoxin-B action. In a Ca2+-free solution, zooxanthellatoxin-B caused neither the contraction nor the increase in cytosolic Ca2+. Furthermore, both pre- and post-treatment with verapamil, a voltage-operated Ca2+-channel blocker, partially suppressed both an increase in cytosolic Ca2+and the contraction by zooxanthellatoxin-B. Zooxanthellatoxin-B-induced contraction was also inhibited by other voltage-operated Ca2+-channel blockers: nifedipine or diltiazem. These results suggest that zooxanthellatoxin-B-elicited contraction is caused by a Ca2+influx into the smooth muscle cells, partially via voltage-operated Ca2+channels.Key words: zooxanthellatoxin, Ca2+imaging, rabbit aorta, contraction, voltage-operated Ca2+-channels.

Effects of potassium channel opener KRN4884 on human conduit arteries used as coronary bypass grafts

AIMS

The effects of a new potassium channel opener KRN4884 on human arteries have not been studied. This study was designed to investigate the effects of KRN4884 on the human internal mammary artery (IMA) in order to provide information on possible clinical applications of KRN4884 for preventing and relieving vasospasm of arterial grafts in coronary artery bypass grafting.

METHODS

IMA segments (n = 140) taken from patients undergoing coronary surgery were studied in the organ chamber. Concentration-relaxation curves for KRN4884 were established in the IMA precontracted with noradrenaline (NA), 5-hydroxytryptamine (5-HT), angiotensin II (ANG II), and endothelin-1 (ET-1). The effect of glibenclamide (GBC) on the KRN4884-induced relaxation was also examined in NA or 5-HT-precontracted IMA. Concentration-contraction curves for the four vasoconstrictors were constructed without/with pretreatment of KNR4884 (1 or 30 microM) for 15 min.

RESULTS

KRN4884 induced less relaxation (P < 0.05) in the precontraction induced by ET-1 (72.9 +/- 5.5%) than by ANG II (94.2 +/- 3.2%) or NA (93.7 +/- 4.1%) with lower EC50 (P < 0.05) for ANG II (-8.54 +/- 0.54 log M) than that for NA (-6.14 +/- 0.15 log M) or ET-1 (-6.69 +/- 0.34 log M). The relaxation in the IMA pretreated with GBC was less than that in control (P < 0.05). KRN4884-pretreatment significantly reduced the contraction (P < 0.05) induced by NA (151.3 +/- 18.4% vs 82.7 +/- 8. 7%), 5-HT (82.7 +/- 12.2% vs 30.1 +/- 7.3%), and ANG II (24.3 +/- 6. 3% vs 5.4 +/- 1.6%), but did not significantly reduce the contraction induced by ET-1 (P > 0.05).

CONCLUSION

KRN4884 has marked vasorelaxant effects on the human IMA contracted by a variety of vasoconstrictors and the effect is vasoconstrictor-selective.

Metabolic dependency of ionophore A23187-induced contraction of ileal longitudinal smooth muscle

1. Tonic contraction in response to K+ is well known to be highly dependent on aerobic metabolism in ileal muscle. The ionophore A23187 (10(-5) M) induced an initial contraction and successive rhythmic contraction in ileal muscle, but did not induce tonic contraction. This study, therefore, was performed to investigate the metabolic dependency during contraction induced by A23187. 2. Under hypoxic conditions, A23187 (10(-5) M) induced an initial contraction accompanied by an increase in lactate release. However, it induced only small rhythmic contractions and decreases in ATP, phosphocreatinine (PCr) and glycogen contents. 3. In glucose-free medium, A23187 (10(-5) M) induced an initial contraction and concomitant significant decrease in the ATP and glycogen contents. However, it did not induce successive rhythmic contractions. In 'glycogen-depleted muscle' stimulated repeatedly with 60 mM K+ in glucose-free medium, 60 mM K+ induced a phasic contraction without tonic contraction. A23187 induced no contraction under these conditions. 4. These results suggested that the initial contraction induced by A23187 was dependent on endogenous glycogen, while successive rhythmic contractions were dependent on aerobic metabolism after supply of external glucose to the ileal muscle.

Effects of the K+ channel opener KRN4884 on the cardiovascular metabolic syndrome model in rats

We examined the effects of the potassium channel opener KRN4884 (5-amino-N-[2-(2-chlorophenyl)ethyl]-N'-cyano-3-pyridinecarboxamidine ) on cardiovascular metabolic syndrome (i.e., syndrome X), in rats. High-fructose diet rats developed hypertension, hypertriglyceridemia, increased total cholesterol/HDL (high-density lipoprotein)-cholesterol ratio, and hyperinsulinemia, KRN4884 (0.3-3.0 mg/kg, twice a day for 14 days, p.o.) alleviated the risk factors in fructose-fed rats. Furthermore, fructose-fed rats exhibited impairment of glucose tolerance and excess insulin secretion when loaded with glucose orally. Treatment with KRN4884 (1.0 mg/kg, twice a day for 14 days, p.o.) improved the glucose intolerance and inhibited hypersecretion of insulin in the glucose-loaded, fructose-fed rats. In contrast, KRN4884 (0.3-1.0 mg/kg, twice a day for 10 days, p.o.) did not affect serum triglyceride, cholesterol, glucose, or insulin concentrations in normal rats. LPL (lipoprotein lipase) activities in skeletal muscle and adipose tissue, and HTGL (hepatic triglyceride lipase) activity in liver were measured after administration of KRN4884 or vehicle twice a day for 14 days in fructose-fed rats. KRN4884 caused a significant increase in LPL activity in muscle and tended to increase LPL activity in adipose tissue in fructose-fed rats. HTGL was decreased in fructose-fed rats as compared with normal controls and was unaffected by KRN4884. These findings suggested that KRN4884 enhances insulin sensitivity and LPL activity, which are related to glucose and lipid metabolism and may be useful for the treatment of syndrome X.

Protective effect of the K+ channel opener KRN4884 on peripheral occlusive arterial disease in rats

1. The effect of the potassium channel opener KRN4884 on the peripheral arterial occlusion model induced by laurate was examined and compared with that of beraprost sodium and nilvadipine. 2. KRN4884 or beraprost sodium prevented macroscopic changes in the paw after the injection of laurate. In contrast, nilvadipine did not improve the lesions. 3. KRN4884 produced a dose-dependent increase in gastrocnemius blood flow in the chronic femoral artery-ligated rats. The effect of KRN4884 on the blood flow was stronger in the hypoxic muscle than in the normal muscle. 4. KRN4884 did not have a direct antiplatelet aggregation activity. 5. These findings suggest that KRN4884 is useful for the therapy of peripheral arterial occlusive disease and that the effect of KRN4884 is associated with an increase in blood flow in ischemic skeletal muscle.

Characteristics of the in vitro vasoactivity of beta-amyloid peptides

The beta-amyloid (A beta 1-40) peptide has previously been shown to enhance phenylephrine contraction of aortic rings in vitro. We have employed a novel observation, that A beta peptides enhance endothelin-1 (ET-1) contraction, to examine the relationship between vasoactivity and potential amyloidogenicity of A beta peptides, the role played by free radicals and calcium in the vasoactive mechanism, and the requirement of an intact endothelial layer for enhancement of vasoactivity. Rings of rat aortae were constricted with ET-1 before and after addition of amyloid peptide and/or other compounds, and a comparison was made between post- and pre-treatment contractions. In this system, vessel constriction is consistently dramatically enhanced by A beta 1-40, is enhanced less so by A beta 1-42, and is not enhanced by A beta 25-35. The endothelium is not required for A beta vasoactivity, and calcium channel blockers have a greater effect than antioxidants in blocking enhancement of vasoconstriction by A beta peptides. In contrast to A beta-induced cytotoxicity, A beta-induced vasoactivity is immediate, occurs in response to low doses of freshly solubilized peptide, and appears to be inversely related to the amyloidogenic potential of the A beta peptides. We conclude that the mechanism of A beta vasoactivity is distinct from that of A beta cytotoxicity. Although free radicals appear to modulate the vasoactive effects, the lack of requirement for endothelium suggests that loss of the free radical balance (between NO and O2-) may be a secondary influence on A beta enhancement of vasoconstriction. These effects of A beta on isolated vessels, and reported effects of A beta in cells of the vasculature, suggest that A beta-induced disruption of vascular tone may be a factor in the pathogenesis of cerebral amyloid angiopathy and Alzheimer's disease. Although the mechanism of enhanced vasoconstriction is unknown, it is reasonable to propose that in vivo contact of A beta peptides with small cerebral vessels may increase their tendency to constrict and oppose their tendency to relax. The subclinical ischemia resulting from this would be expected to up-regulate beta APP production in and around the vasculature with further increase in A beta formation and deposition. The disruptive and degenerative effects of such a cycle would lead to the complete destruction of cerebral vessels and consequently neuronal degeneration in the affected areas.

Resurgence of carbon monoxide: an endogenous gaseous vasorelaxing factor

Carbon monoxide (CO) is an endogenously generated gas that may play an important physiological role in the regulation of vascular tone. The CO-induced vasorelaxation, as a result of a direct action on vascular smooth muscles, has been demonstrated in many cases. Three major cellular mechanisms are proposed to explain the vasorelaxing effect of CO. These include the activation of soluble guanylyl cyclase, stimulation of various types of K channels, and inhibition of the cytochrome P450 dependent monooxygenase system in vascular smooth muscle cells. An interaction between CO and nitric oxide may also significantly contribute to the fine tuning of vascular tone. Furthermore, alterations in either the endogenous production of CO or the vascular responsiveness to CO have been encountered in several pathophysiological situations. A better understanding of the vascular effects of CO and the underlying cellular and molecular mechanisms will pave the way for the establishment of the role played by CO in vascular physiology and pathophysiology.Key words: carbon monoxide, heme oxygenase, smooth muscles, vasorelaxation.