K(ATP) channels and pancreatic islet blood flow in anesthetized rats: increased blood flow induced by potassium channel openers

It's What and When You Eat: An Overview of Transcriptional and Epigenetic Responses to Dietary Perturbations in Pancreatic Islets

Our ever-changing modern environment is a significant contributor to the increased prevalence of many chronic diseases, and particularly, type 2 diabetes mellitus (T2DM). Although the modern era has ushered in numerous changes to our daily living conditions, changes in "what" and "when" we eat appear to disproportionately fuel the rise of T2DM. The pancreatic islet is a key biological controller of an organism's glucose homeostasis and thus plays an outsized role to coordinate the response to environmental factors to preserve euglycemia through a delicate balance of endocrine outputs. Both successful and failed adaptation to dynamic environmental stimuli has been postulated to occur due to changes in the transcriptional and epigenetic regulation of pathways associated with islet secretory function and survival. Therefore, in this review we examined and evaluated the current evidence elucidating the key epigenetic mechanisms and transcriptional programs underlying the islet's coordinated response to the interaction between the timing and the composition of dietary nutrients common to modern lifestyles. With the explosion of next generation sequencing, along with the development of novel informatic and -omic approaches, future work will continue to unravel the environmental-epigenetic relationship in islet biology with the goal of identifying transcriptional and epigenetic targets associated with islet perturbations in T2DM.

Pancreatic Blood Flow with Special Emphasis on Blood Perfusion of the Islets of Langerhans

The pancreatic islets are more richly vascularized than the exocrine pancreas, and possess a 5- to 10-fold higher basal and stimulated blood flow, which is separately regulated. This is reflected in the vascular anatomy of the pancreas where islets have separate arterioles. There is also an insulo-acinar portal system, where numerous venules connect each islet to the acinar capillaries. Both islets and acini possess strong metabolic regulation of their blood perfusion. Of particular importance, especially in the islets, is adenosine and ATP/ADP. Basal and stimulated blood flow is modified by local endothelial mediators, the nervous system as well as gastrointestinal hormones. Normally the responses to the nervous system, especially the parasympathetic and sympathetic nerves, are fairly similar in endocrine and exocrine parts. The islets seem to be more sensitive to the effects of endothelial mediators, especially nitric oxide, which is a permissive factor to maintain the high basal islet blood flow. The gastrointestinal hormones with pancreatic effects mainly influence the exocrine pancreatic blood flow, whereas islets are less affected. A notable exception is incretin hormones and adipokines, which preferentially affect islet vasculature. Islet hormones can influence both exocrine and endocrine blood vessels, and these complex effects are discussed. Secondary changes in pancreatic and islet blood flow occur during several conditions. To what extent changes in blood perfusion may affect the pathogenesis of pancreatic diseases is discussed. Both type 2 diabetes mellitus and acute pancreatitis are conditions where we think there is evidence that blood flow may contribute to disease manifestations. © 2019 American Physiological Society. Compr Physiol 9:799-837, 2019.

Antiarrhythmic activity of a new spiro-cyclic benzopyran activator of the cardiac mitochondrial ATP dependent potassium channels

'Compound A' (4(ı)-(N-(4-acetamidobenzyl))-2,2-dimethyl-2,3-dihydro-5(ı)H-spiro[chromene-4,2(ı)-[1,4]oxazinan]-5(ı)-one) is a new spiro-cyclic benzopyran activator of the mitochondrial ATP-dependent potassium channels (mitoKATP). We researched the effect of compound A on ischemia/reperfusion (I/R)-induced ventricular arrhythmias. We also tested the hypothesis that the application of the activation of mitoKATP in combination with the inhibition of sarcolemmal ATP-dependent potassium channels (sarcKATP) may produce a stronger antiarrhythmic effect. In anesthetized rats, myocardial ischemia was performed by ligating the left main coronary artery followed by reperfusion. At a dose of 10 mg/kg, compound A significantly decreased arrhythmia scores and the total length of arrhythmias, whereas this was found to be ineffective at a dose of 3 mg/kg. Pre-treatment with 5-HD, a selective mitoKATP blocker, abolished the antiarrhythmic effect of compound A. Both diazoxide, a selective mitoKATP opener and HMR 1098, a selective sarcKATP blocker, significantly decreased the total length of arrhythmias. However, the combination of neither diazoxide nor compound A with HMR 1098 showed no additional therapeutic benefit. These results reveal that compound A may have a dose-dependent antiarrythmic effect, which is more pronounced than the antiarrhythmic effect of diazoxide. The antiarrhythmic effect of compound A may possibly depend on mitoKATP activation.

Diazoxide increases liver and kidney HSP25 and HSP70 after shock and stroke

BACKGROUND

The compound, diazoxide (DZ), is known to induce preconditioning through its effect as a mitochondrial K(ATP) channel opener and succinate dehydrogenase inhibitor. Our team tested the hypothesis that pharmacological induction of ischemic preconditioning with DZ can offer cytoprotection and preserve vital tissues after hemorrhagic shock and stroke.

MATERIALS AND METHODS

Sprague-Dawley male rats received an intraperitoneal injection of sterile saline or 5 mg/kg DZ in saline 24 h prior to 1 h of hemorrhagic shock, by approximately 40% total blood loss volume (Shock Study), or a permanent unilateral common carotid ligation just before shock (Stroke + Shock Study). While remaining under isoflurane anesthesia, animals then received 81 mL/kg intravenous sterile saline over the next 45 min for recovery and survived for another 24 h.

RESULTS

When DZ was administered 24 h prior to shock, it significantly reduced hyperglycemia, which in vehicle-treated animals persisted after resuscitation. DZ also attenuated hyperlactatemia during the 1-h shock period. With more severe trauma from combined stroke and shock, DZ also decreased hyperlactatemia and hyperglycemia levels but the reduction was only significant for hyperglycemia. The expression levels of heat shock proteins 25 (HSP25) and 70 (HSP70) were used as biomarkers for response of the kidney and liver to DZ and combined stroke and shock. Compared to vehicle-treated animals, DZ-treated rats subjected to shock and stroke exhibited increased HSP25 and HSP70 in kidney and liver tissue.

CONCLUSIONS

DZ-attenuated physiological indicators of metabolic stress following shock or combined shock and stroke and enhanced the up-regulation of cytoprotective heat shock protein expression.

Islet Vasculature as a Regulator of Endocrine Pancreas Function

The islets of Langerhans consist of endocrine cells embedded in a network of specialized capillaries that regulate islet blood flow. Despite evidence for a critical role of islet perfusion in endocrine pancreas function, there is information to support no fewer than three models of endocrine cell perfusion, emphasizing the lack of a universally accepted physiological theory. Islet blood flow is regulated by signals, such as hormones and nutrients that reach the islet vasculature from distant tissues via the bloodstream. In addition, islet perfusion determines communication between endocrine and exocrine cells and between different types of endocrine cells within islets. Interest in islet microcirculation has increased after improvements in islet transplantation, a therapy for diabetes mellitus that requires revascularization of grafted islets in a new host organ. Abnormal revascularization is thought to be partly responsible for differences in graft and native islet function. Similarly, angiogenesis has been shown to be a critical step in the transformation of islet hyperplasia to neoplasia.

Nateglinide, a non-sulfonylurea rapid insulin secretagogue, increases pancreatic islet blood flow in rats

We studied whether the rapid hypoglycemic action of nateglinide is associated with an increase in islet blood flow. Islet blood flow was measured using the two-colour microsphere method. Orally administered nateglinide with glucose acutely increased islet blood flow to levels greater than those after glucose alone or tolbutamide with glucose in conscious Sprague-Dawley rats (percent increase at 10 min after oral administration; nateglinide+glucose, 125+/-25%; glucose, 33+/-11%, p<0.001; tolbutamide+glucose, 42+/-23%, p<0.01). Nateglinide administered with non-metabolisable 3-O-methylglucose also increased islet blood flow (61+/-17%). The stimulated islet blood flow significantly correlated with serum insulin levels. N(G)-monomethyl-L-arginine, a nitric oxide synthase inhibitor, completely inhibited the increase in islet blood flow induced by nateglinide with glucose. Intravenously administered nateglinide did not significantly affect the already increased islet blood flow in diabetic Otsuka Long-Evans Tokushima Fatty rats. Our results indicated that nateglinide acutely increased islet blood flow at least in part through a nitric oxide-dependent mechanism.

Novel and Established Potassium Channel Openers Stimulate Hair Growth In Vitro: Implications for their Modes of Action in Hair Follicles

Although ATP-sensitive potassium (K(ATP)) channel openers, e.g., minoxidil and diazoxide, can induce hair growth, their mechanisms require clarification. Improved drugs are needed clinically. but the absence of a good bioassay hampers research. K(ATP) channels from various tissues contain subtypes of the regulatory sulfonylurea receptor, SUR, and pore-forming, K(+) inward rectifier subunits, Kir6.X, giving differing sensitivities to regulators. Therefore, the in vitro effects of established potassium channel openers and inhibitors (tolbutamide and glibenclamide), plus a novel, selective Kir6.2/SUR1 opener, NNC 55-0118, were assessed on deer hair follicle growth in serum-free median without streptomycin. Minoxidil (0.1-100 microM, p<0.001), NNC 55-0118 (1 mM, p<0.01; 0.1, 10, 100 microM, p<0.001), and diazoxide (10 microM, p<0.01) increased growth. Tolbutamide (1 mM) inhibited growth (p<0.001) and abolished the effect of 10 microM minoxidil, diazoxide and NNC 55-0118; glibenclamide (10 microM) had no effect, but prevented stimulation by 10 microM minoxidil. Phenol red stimulated growth (p<0.001), but channel modulator responses remained unaltered. Thus, deer follicles offer a practical, ethically advantageous in vitro bioassay that reflects clinical responses in vivo. The results indicate direct actions of K(ATP) channel modulators within hair follicles via two types of channels, with SUR 1 and SUR 2, probably SUR2B, sulfonylurea receptors.