No abstract

No abstract

The sulphonylurea gliclazide is widely used in the treatment of Type II (non-insulin-dependent) diabetes mellitus because of its ability to stimulate insulin secretion from pancreatic beta-cells. Like other sulphonylureas, its principal target is the ATP-sensitive potassium (K ATP ) channel. This channel plays a major role in controlling the beta-cell membrane potential and thereby insulin release. At low plasma glucose concentrations, K ATP channels are open and the resulting K + efflux holds the beta-cell membrane at a hyperpolarized potential. Closure of K ATP channels by glucose or sulphonylureas causes depolarization of the beta-cell membrane, leading to opening of voltage-gated Ca 2+ channels, Ca 2+ influx and a rise in intracellular Ca 2+ which stimulates the exocytosis of insulin-containing secretory granules [1].K ATP channels are also found in a variety of other cell types including cardiac, smooth and skeletal muscle, and some brain neurones [2]. Although their roles in extrapancreatic tissues are less well characterised, it is likely that they open in response to metabolic stress, such as during cardiac and cerebral is- Abstract Aims/hypothesis. Sulphonylureas stimulate insulin secretion by closing ATP-sensitive potassium (K ATP ) channels in the pancreatic beta-cell membrane. K ATP channels are also found in other tissues, including heart and smooth muscle, where they link cellular metabolism to electrical activity. The sulphonylurea gliclazide blocks recombinant beta-cell K ATP channels (Kir6.2/ SUR1) but not heart (Kir6.2/SUR2A) or smooth muscle (Kir6.2/SUR2B) K ATP channels with high potency. In this study, we examined the specificity of gliclazide for the native (as opposed to recombinant) K ATP channels in beta cells, heart and smooth muscle. Methods. The action of the drug was studied by whole-cell current recordings of native K ATP channels in isolated pancreatic beta-cells and myocytes from heart and smooth muscle. Results. Gliclazide blocked whole-cell beta-cell K ATP currents with an IC 50 of 184 30 nmol/l (n = 6±10) but was much less effective in cardiac and smooth muscle (IC 50 s of 19.5 5.4 mmol/l (n = 6±12) and 37.9 1.0 mmol/l (n = 5±10), respectively). In all three tissues, the action of the drug on whole-cell K ATP currents was rapidly reversible. In inside-out patches on beta-cells, gliclazide (1 mmol/l) produced a maximum of 66 13 % inhibition (n = 5), compared with more than 98 % block in the whole-cell configuration. Conclusion/interpretation. Gliclazide is a high-potency sulphonylurea which shows specificity for the pancreatic beta-cell K ATP channel over heart and smooth muscle. In this respect, it differs from glibenclamide. The difference in the maximal block observed in the excised patch and whole-cell recordings from betacells, may be due to the absence of intracellular Mgnucleotides in the excised patch experiments. [Diabetologia (2001

1 The K ATP channel opener diazoxide has been proposed to protect cardiac muscle against ischaemia by opening mitochondrial K ATP channels to depolarize the mitochondrial membrane potential, DC m . We have used the¯uorescent dye TMRE to measure DC m in adult rat freshly isolated cardiac myocytes exposed to diazoxide and metabolic inhibition. 2 Diazoxide, at concentrations that are highly cardioprotective (100 or 200 mM), caused no detectable increase in TMRE¯uorescence (n=27 cells). However, subsequent application of the protonophore FCCP, which should collapse DC m , led to large increases in TMRE¯uorescence (4300%). 3 Metabolic inhibition (MI: 2 mM NaCN+1 mM iodoacetic acid (IAA) led to an immediate partial depolarization of DC m , followed after a few minutes delay by complete depolarization which was correlated with rigor contracture. Removal of metabolic inhibition led to abrupt mitochondrial repolarization followed in many cells by hypercontracture, indicated by cell rounding and loss of striated appearance. 4 Prior application of diazoxide (100 mM) reduced the number of cells that hypercontracted after metabolic inhibition from 63.7+4.7% to 24.2+1.8% (P50.0001). 5-hydroxydeanoate (100 mM) reduced the protection of diazoxide (46.8+2.7% cells hypercontracted, P50.0001 vs diazoxide alone). 5 Diazoxide caused no detectable change in¯avoprotein auto¯uorescence (n=26 cells). 6 Our results suggest that mitochondrial depolarization and¯avoprotein oxidation are not inevitable consequences of diazoxide application in intact cardiac myocytes, and that they are also not essential components of the mechanism by which it causes protection.

No abstract

The effects of removing extracellular Ca2+ and Mg2+ on the membrane potential, membrane current and intracellular Na+ activity (aiNa) were investigated in guinea-pig and rat ventricular myocytes. Membrane potential was recorded with a patch pipette and whole-cell membrane currents using a single-electrode voltage clamp. Both guinea-pig and rat cells depolarize when the bathing Ca2+ and Mg2+ are removed and the steady-state aiNa increases rapidly from a resting value of 6.4+/- 0.6 mM to 33+/-3.8 mM in guinea-pig (n=9) and from 8.9+/-0.8 mM to 29.3+/-3.0 mM (n=5) in rat ventricular myocytes. Guinea-pig myocytes partially repolarized when, in addition to removal of the bathing Ca2+ and Mg2+, K+ was also removed, however rat cells remained depolarized. A large diltiazem-sensitive inward current was recorded in guinea-pig and rat myocytes, voltage-clamped at -20 mV, when the bathing divalent cations were removed. When the bathing K+ was removed after Ca2+ and Mg2+ depletion, a large outward K+ current developed in guinea-pig, but not in rat myocytes. This current had a reversal potential of -80+/-0.7 mV and was not inhibited by high Mg2+ or glybenclamide indicating that it is not due to activation of non-selective cation or adenosine triphosphate (ATP)-sensitive K channels. The current was not activated when Li+ replaced the bathing Na+ and was blocked by R-56865, suggesting that it was due to the activation of KNa channels.

No abstract

1 Women are more prone to develop torsades de pointes, a rare life-threatening polymorphic ventricular tachycardia, than are men during administration of medicines that have the potential to block I Kr (rapid delayed rectifier cardiac K þ current) and to prolong the QT interval. Blockade of I Kr , hypokalaemia and extreme bradycardia were used to evaluate whether there are gender differences in cardiac repolarisation in canine Purkinje fibres (PFs). Microelectrode techniques were employed to measure action potential (AP) parameters in PFs from adult female and male dogs. 2 Under control conditions, fibres from female animals in normal or low K þ conditions exhibited significantly longer AP durations at 50% (APD 50 ) and 90% (APD 90 ) of repolarisation as compared with APDs of fibres from male animals. 3 Gender-related difference to rate adaptation was also present in APD 90 of fibres from female animals compared to males. 4 At a stimulation rate of 0.2 Hz, but not at 1.0 Hz, dofetilide elicited a significantly higher increase in APD 90 , incidence of early afterdepolarisations, triggered and sustained-triggered activities (TAs) in fibres from female animals compared to males in either normal or low K þ conditions. The sustained TAs were reversed by raising the concentration of [K þ ] 0 in Purkinje preparations from both male (one out of one) and female (12 out of 12) dogs. 5 In conclusion, our data provide experimental evidence pointing to gender differences in canine AP repolarisation. PFs from female dogs can be used in safety pharmacology studies as a sensitive model for evaluating the potential proarrhythmic events in vitro of a new medicinal product. British Journal of Pharmacology (2004)