Efonidipine Hydrochloride: A Dual Blocker of L‐ and T‐Type Ca2+ Channels

T‐type Ca²⁺ channels have properties different from those of the L‐type and are involved in cardiac pacemaking and regulation of blood flow, but not in myocardial contraction. Efonidipine is an antihypertensive and antianginal drug with dihydropyridine structure that was recently found to block both L‐ and T‐type Ca²⁺ channels. In isolated myocardial and vascular preparations, efonidipine has potent negative chronotropic and vasodilator effects but only a weak negative inotropic effect. In experimental animals and patients, reduction of blood pressure by the drug was accompanied by no or minimum reflex tachycardia leading to improvement of myocardial oxygen balance and maintenance of cardiac output. Efonidipine increased glomerular filtration rate without increasing intraglomerular pressure. By relaxing both the afferent and efferent arterioles, efonidipine markedly reduced proteinuria. Thus, efonidipine, an L‐ and T‐type dual Ca²⁺ channel blocker, appears to have an ideal profile as an antihypertensive and antianginal drug with organ‐protective effects in the heart and kidney.     

K+-induced dilation of hamster cremasteric arterioles involves both the Na+/K+-ATPase and inward-rectifier K+ channels

OBJECTIVE: The mechanism by which elevated extracellular potassium ion concentration ([K+]o) causes dilation of skeletal muscle arterioles was evaluated. METHODS: Arterioles (n = 111) were hand-dissected from hamster cremaster muscles, cannulated with glass micropipettes and pressurized to 80 cm H2O for in vitro study. The vessels were superfused with physiological salt solution containing 5 mM KCl, which could be rapidly switched to test solutions containing elevated [K+]o and/or inhibitors. The authors measured arteriolar diameter with a computer-based diameter tracking system, vascular smooth muscle cell membrane potential with sharp micropipettes filled with 200 mM KCl, and changes in intracellular Ca2+ concentration ([Ca2+]i) with Fura 2. Membrane currents and potentials also were measured in enzymatically isolated arteriolar muscle cells using patch clamp techniques. The role played by inward rectifier K+ (KIR) channels was tested using Ba2+ as an inhibitor. Ouabain and substitution of extracellular Na+ with Li+ were used to examine the function of the Na+/K+ ATPase. RESULTS: Elevation of [K+]o from 5 mM up to 20 mM caused transient dilation of isolated arterioles (27 +/- 1 microm peak dilation when [K+]o was elevated from 5 to 20 mM, n = 105, p <.05). This dilation was preceded by transient membrane hyperpolarization (10 +/-1 mV, n = 23, p <.05) and by a fall in [Ca2+]i as indexed by a decrease in the Fura 2 fluorescence ratio of 22 +/- 5% (n = 4, p <.05). Ba(2+) (50 or 100 microM) attenuated the peak dilation (40 +/- 8% inhibition, n = 22) and hyperpolarization (31 +/- 12% inhibition, n = 7, p <.05) and decreased the duration of responses by 37 +/-11% (n = 20, p < 0.05). Both ouabain (1 mM or 100 microM) and replacement of Na+ with Li+ essentially abolished both the hyperpolarization and vasodilation. CONCLUSIONS: Elevated [K+]o causes transient vasodilation of skeletal muscle arterioles that appears to be an intrinsic property of the arterioles. The results suggest that K+-induced dilation involves activation of both the Na+/K+ ATPase and KIR channels, leading to membrane hyperpolarization, a fall in [Ca2+]i, and culminating in vasodilation. The Na+/K+ ATPase appears to play the major role and is largely responsible for the transient nature of the response to elevated [K+]o, whereas KIR channels primarily affect the duration and kinetics of the response.     

Fructose induces glucose‐dependent insulinotropic polypeptide,glucagon‐like peptide‐1 and insulin secretion: Role of adenosine triphosphate‐sensitive K+ channels

Adenosine triphosphate-sensitive K⁺ (K_ATP) channels play an essential role in glucose-induced insulin secretion from pancreatic β-cells. It was recently reported that the K_ATP channel is also found in the enteroendocrine K-cells and L-cells that secrete glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), respectively. In the present study, we investigated the involvement of the K_ATP channel in fructose-induced GIP, GLP-1 and insulin secretion in mice. Fructose stimulated GIP secretion, but pretreatment with diazoxide, a K_ATP channel activator, did not affect fructose-induced GIP secretion under streptozotocin-induced hyperglycemic conditions. Fructose significantly stimulated insulin secretion in Kir6.2^+/+ mice, but not in mice lacking K_ATP channels (Kir6.2^−/−), and fructose stimulated GLP-1 secretion in both Kir6.2^+/+ mice and Kir6.2^−/− mice under the normoglycemic condition. In addition, diazoxide completely blocked fructose-induced insulin secretion in Kir6.2^+/+ mice and in MIN6-K8 β-cells. These results show that fructose-induced GIP and GLP-1 secretion is K_ATP channel-independent and that fructose-induced insulin secretion is K_ATP channel-dependent.     

Na+K+-Atpase Activity and Responsiveness of Vascular Smooth Muscle to Norepinephrine,Angiotensin II and Calcium Ionophore A23187 in Guinea Pig Aortic Strips

The functional significance of the Na⁺K⁺-ATPase activity in defining the sensitivity of vascular smooth muscle response to pressor stimuli was studied in guinea pig aortic strips. Subthreshold doses of ouabain (10^-8, 10^-7, 10^-6M), potentiated the norepinephrine- and angiotensin II-induced contractile responses, dose-dependently. Furthermore, in the presence of subthreshold dose of ouabain (10^-6M), tension developments were observed with subthreshold doses of norepinephrine and angiotensin II. The mechanism by which subthreshold dose of ouabain potentiated the norepinephrine-induced contractile response was revealed to involve the enhancement both of sensitivity and contractile activity. Ouabain (10^-6M) potentiated the norepinephrine- and A23187-induced contractile responses, even in the presence of verapamil. These facts indicate that suppression of the vascular Na⁺K⁺-ATPase activity could favor the development of hypertension through potentiating contractile responses to various stimuli and that the potentiation could be a reflection, at least partly, of the decrease in Ca²⁺-efflux     

Rebaudioside A directly stimulates insulin secretion from pancreatic beta cells: a glucose-dependent action via inhibition of ATP-sensitive K-channels

Recently, we showed that rebaudioside A potently stimulates the insulin secretion from isolated mouse islets in a dose-, glucose- and Ca(2+)-dependent manner. Little is known about the mechanisms underlying the insulinotropic action of rebaudioside A. The aim of this study was to define the signalling system by which, rebaudioside A acts. Isolated mouse islets were used in the cAMP[(125)I] scintillation proximity assay to measure total cAMP level, and in a luminometric method to measure intracellular ATP and ADP concentrations. Conventional and permeabilized whole-cell configuration of the patch-clamp technique was used to verify the effect of rebaudioside A on ATP-sensitive K(+)-channels from dispersed single beta cells from isolated mouse islets. Insulin was measured by radioimmunoassay from insulinoma MIN6 cells. In the presence of 16.7 mM glucose, the addition of the maximally effective concentration of rebaudioside A (10(-9) M) increased the ATP/ADP ratio significantly, while it did not change the intracellular cAMP level. Rebaudioside A (10(-9) M) and stevioside (10(-6) M) reduced the ATP-sensitive potassium channel (K(ATP)) conductance in a glucose-dependent manner. Moreover, rebaudioside A stimulated the insulin secretion from MIN6 cells in a dose- and glucose-dependent manner. In conclusion, the insulinotropic effect of rebaudioside A is mediated via inhibition of ATP-sensitive K(+)-channels and requires the presence of high glucose. The inhibition of ATP-sensitive K(+)-channels is probably induced by changes in the ATP/ADP ratio. The results indicate that rebaudioside A may offer a distinct therapeutic advantage over sulphonylureas because of less risk of causing hypoglycaemia.     

The super sickling haemoglobin HbS‐Oman: a study of red cell sickling,K+ permeability and associations with disease severity in patients heterozygous for HbA and HbS‐Oman (HbA/S‐Oman genotype)

Studying different sickle cell genotypes may throw light on the pathogenesis of sickle cell disease (SCD). Here, the clinical profile, red cell sickling and K⁺ permeability in 29 SCD patients (15 patients with severe disease and 14 with a milder form) of HbA/S‐Oman genotype were analysed. The super sickling nature of this Hb variant was confirmed. The red cell membrane permeability to K⁺ was markedly abnormal with elevated activities of P_sickle, Gardos channel and KCl cotransporter (KCC). Results were consistent with Ca²⁺ entry and Mg²⁺ loss via P_sickle stimulating Gardos channel and KCC activities. The abnormal red cell behaviour was similar to that in the commonest genotype of SCD, HbSS, in which the level of mutated Hb is considerably higher. Although activities of all three K⁺ transporters also correlated with the level of HbS‐Oman, there was no association between transport phenotype and disease severity. The super sickling behaviour of HbS‐Oman may obviate the need for solute loss and red cell dehydration to encourage Hb polymerisation, required in other SCD genotypes. Disease severity was reduced by concurrent α thalassaemia, as observed in other SCD genotypes, and represents an obvious genetic marker for prognostic tests of severity in young SCD patients of the HbA/S‐Oman genotype.