Inhibition by protein kinase C of the 86Rb+ efflux and vasorelaxation induced by P1075, a K(ATP) channel opener, in rat isolated aorta

Exploring the Pathophysiology of ATP-Dependent Potassium Channels in Insulin Resistance

Ionic channels are present in eucaryotic plasma and intracellular membranes. They coordinate and control several functions. Potassium channels belong to the most diverse family of ionic channels that includes ATP-dependent potassium (KATP) channels in the potassium rectifier channel subfamily. These channels were initially described in heart muscle and then in other tissues such as pancreatic, skeletal muscle, brain, and vascular and non-vascular smooth muscle tissues. In pancreatic beta cells, KATP channels are primarily responsible for maintaining the membrane potential and for depolarization-mediated insulin release, and their decreased density and activity may be related to insulin resistance. KATP channels' relationship with insulin resistance is beginning to be explored in extra-pancreatic beta tissues like the skeletal muscle, where KATP channels are involved in insulin-dependent glucose recapture and their activation may lead to insulin resistance. In adipose tissues, KATP channels containing Kir6.2 protein subunits could be related to the increase in free fatty acids and insulin resistance; therefore, pathological processes that promote prolonged adipocyte KATP channel inhibition might lead to obesity due to insulin resistance. In the central nervous system, KATP channel activation can regulate peripheric glycemia and lead to brain insulin resistance, an early peripheral alteration that can lead to the development of pathologies such as obesity and Type 2 Diabetes Mellitus (T2DM). In this review, we aim to discuss the characteristics of KATP channels, their relationship with clinical disorders, and their mechanisms and potential associations with peripheral and central insulin resistance.

Deterioration of the protein kinase C-K(ATP) channel pathway in regulation of coronary flow in hypercholesterolaemic rabbits

We studied the effect of experimental hypercholesterolaemia/atherosclerosis on changes in coronary flow and cardiac function, induced by protein kinase C and ATP-sensitive K(+) (K(ATP)) channel modulators in isolated Langendorff-perfused rabbit hearts. Both phorbol 12-myristate-13-acetate (PMA) and phorbol 12,13-dibutyrate (PDB, 0.1 microM each), activators of protein kinase C, decreased, whereas staurosporine, (0.1 microM), a protein kinase C inhibitor, increased coronary flow and left ventricular dP/dt, an index of ventricular contractility. Glyburide (5-50 microM), a K(ATP) channel inhibitor, blocked the effect of staurosporine. The phorbol esters were without effect in the presence of pinacidil (5 microM), a K(ATP) channel activator. Neither the protein kinase C modulators nor glyburide produced any effect on coronary flow and left ventricular contractility, when the hearts were prepared from animals either maintained on a cholesterol (1.5%)-enriched diet or treated with lovastatin (5 mg/kg/day per os). Treatment with farnesol (1 mg/kg twice a day for 7 days intravenously) restored the reactivity of hearts from either hypercholesterolaemic or lovastatin-treated animals to protein kinase C modulators. We conclude that non-cholesterol mevalonate products are necessary for the functional integrity of the protein kinase C-K(ATP) channel pathway in the rabbit heart.

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.

Phorbol ester modulation of active ion transport across the rabbit conjunctival epithelium

Protein kinase C (PKC) activation elicits diverse cell-type specific effects on key epithelial transporters. The present work examined the influence of phorbol esters, which are known activators of PKC isoenzymes, on the short-circuit current (Isc), a direct measure of net transcellular electrolyte transport, of the rabbit conjunctiva. In this preparation, the Iscmeasures a Na+-dependent, bumetanide-inhibitable Cl-transport in the basolateral-to-apical direction plus an amiloride-resistant Na+absorptive process in the opposite direction. Additions of phorbol 12-myristate-13-acetate (PMA) to the basolateral bathing media did not affect the transepithelial electrical parameters; but its introduction to the apical bath at 1 and 10 micrometers elicited a transient ( approximately 2 min duration) Iscspike followed by a sustained reduction relative to the control level. Such PMA-elicited Iscreductions were from 14. 0+/-2.0 to 3.1+/-0.8 microA cm-2(+/-s.e.m.'s, n =3) at 1 micrometer and from 16.5+/-1.9 to 4.6+/-0.7 microA cm-2(n =22) at 10 micrometers. The former concentration failed to produce extensive Iscreductions in 3 other experiments. Similar results were obtained with phorbol 12, 13-dibutyrate (PDBu). Its apical administration at 0.1 micrometer reduced the Iscfrom 18.5+/-4.1 to 7.8+/-2.0 (n =3), and from 16. 5+/-2.9 to 6.9+/-1.2 (n =7) when introduced at 1 micrometer. The phorbol-evoked Iscreductions occurred without a simultaneous change in transepithelial resistance (Rt). However, after about 15-20 min, Rtgradually declined by about 25%. In contrast to these results, treatment with a phorbol ester known not to activate PKC (4-alpha-PMA) did not affect the electrical parameters when added at 10 micrometers. PMA- and PDBu-evoked Iscreductions could be obtained with conjunctiva that were (1) pretreated with bumetanide, (2) bathed in Cl--free media, and (3) pretreated with amphotericin B, changes consistent with a likely inhibition of the basolateral Na+/K+pump. Such Iscinhibitions were attenuated with conjunctiva pre-exposed to 1 micrometer staurosporine, a nonselective kinase inhibitor known to suppress PKC activity. Staurosporine, in itself, produced a rapid 26% Iscinhibition (n =15) along with a 17% Rtincrease upon its apical introduction. These electrical responses were less extensive in Cl--free media and absent in amphotericin B-treated conjunctiva, suggesting the presence of a kinase-mediated regulation of apical channels for both Na+and Cl-. Overall, these results imply that in addition to previously demonstrated epinephrine-elicited, up-regulation of Cl-secretion, mechanisms may also exist, via PKC activation, to suppress Na+/K+pumping and consequently reduce transepithelial transport rates.