Hyperosmolality-induced abnormal patterns of calcium mobilization in smooth muscle cells from non-diabetic and diabetic rats

Surprising Hyperkalemia of 10.2 mmol/L in a Patient with Hyperglycemia: A Case Report

Hyperkalemia is a life-threatening condition potentially leading to cardiac arrest. Here, we report a case of surprising severe hyperkalemia of 10.2 mmol/L in a diabetic patient with previously normal kidney function presenting without discernible clinical symptoms to our emergency department. The patient was admitted because of hyperglycemia of 32.8 mmol/L, which was detected during daily testing in her nursing home. The hyperkalemia was caused by prerenal failure due to hyperglycemic polyuria which led to volume depletion, and worsened by a combination of potassium-sparing drugs and potassium supplementation. The patient was treated conservatively. Eighteen hours later, the serum potassium concentration was 4.6 mmol/L. The patient could be released 6 days later. To our knowledge, this is the highest described hyperkalemia treated conservatively and survived without cardiopulmonary resuscitation.

Hyperglycaemia disrupts conducted vasodilation in the resistance vasculature of db/db mice

Vascular dysfunction in small resistance arteries is observed during chronic elevations in blood glucose. Hyperglycaemia-associated effects on endothelium-dependent vasodilation have been well characterized, but effects on conducted vasodilation in the resistance vasculature are not known. Small mesenteric arteries were isolated from healthy and diabetic db/db mice, which were used as a model of chronic hyperglycaemia. Endothelium-dependent vasodilation via the G_q/11-coupled proteinase activated receptor 2 (PAR2) was stimulated with the selective agonist SLIGRL. The Ca²⁺-sensitive fluorescent indicator fluo-8 reported changes in endothelial cell (EC) [Ca²⁺]_i, and triple cannulated bifurcating mesenteric arteries were used to study conducted vasodilation. Chronic hyperglycaemia did not affect either EC Ca²⁺ or local vasodilation to SLIGRL. However, both acute and chronic exposure to high glucose or the mannitol osmotic control attenuated conducted vasodilation to 10μM SLIGRL. This investigation demonstrates for the first time that a hypertonic solution containing glucose or mannitol can interfere with the spread of a hyperpolarizing current along the endothelium in a physiological setting. Our findings reiterate the importance of studying the effects of hyperglycaemia in the vasculature, and provide the basis for further studies regarding the modulation of junctional proteins involved in cell to cell communication by diseases such as diabetes.

Elevated Ca2+ sparklet activity during acute hyperglycemia and diabetes in cerebral arterial smooth muscle cells

Ca(+) sparklets are subcellular Ca(2+) signals produced by the opening of L-type Ca(2+) channels (LTCCs). In cerebral arterial myocytes, Ca(2+) sparklet activity varies regionally, resulting in low and high activity, "persistent" Ca(2+) sparklet sites. Although increased Ca(2+) influx via LTCCs in arterial myocytes has been implicated in the chain of events contributing to vascular dysfunction during acute hyperglycemia and diabetes, the mechanisms underlying these pathological changes remain unclear. Here, we tested the hypothesis that increased Ca(2+) sparklet activity contributes to higher Ca(2+) influx in cerebral artery smooth muscle during acute hyperglycemia and in an animal model of non-insulin-dependent, type 2 diabetes: the dB/dB mouse. Consistent with this hypothesis, acute elevation of extracellular glucose from 10 to 20 mM increased the density of low activity and persistent Ca(2+) sparklet sites as well as the amplitude of LTCC currents in wild-type cerebral arterial myocytes. Furthermore, Ca(2+) sparklet activity and LTCC currents were higher in dB/dB than in control myocytes. We found that activation of PKA contributed to higher Ca(2+) sparklet activity during hyperglycemia and diabetes. In addition, we found that the interaction between PKA and the scaffolding protein A-kinase anchoring protein was critical for the activation of persistent Ca(2+) sparklets by PKA in cerebral arterial myocytes after hyperglycemia. Accordingly, PKA inhibition equalized Ca(2+) sparklet activity between dB/dB and wild-type cells. These findings suggest that hyperglycemia increases Ca(2+) influx by increasing Ca(2+) sparklet activity via a PKA-dependent pathway in cerebral arterial myocytes and contributes to vascular dysfunction during diabetes.

Hypertonicity increases rabbit atrium and pulmonary vein arrhythmogenesis: a potential contributor to the genesis of atrial fibrillation

1. Pulmonary veins are the most important focus for the initiation of atrial fibrillation. Diabetes mellitus may be associated with hypertonicity and increased occurrence of atrial fibrillation. 2. The purpose of the present study was to investigate whether hypertonicity alters the electrophysiological characteristics of pulmonary veins and atria to enhance the genesis of atrial fibrillation. 3. A whole-cell patch-clamp technique was used to investigate action potentials and ionic currents in rabbit isolated single pulmonary vein and atrial cardiomyocytes during immersion in isotonic and hypertonic (1.2x normal osmolality) solutions. 4. Hypertonicity increased the spontaneous beating rates of pulmonary vein cardiomyocytes from 2.3 +/- 0.3 to 3.4 +/- 0.3 Hz (n = 11; P < 0.001). Hypertonicity prolonged action potential duration to a greater extent in atrial cardiomyocytes than in pulmonary vein cardiomyocytes. Compared with atrial cardiomyocytes, hypertonicity increased the transient inward currents and Na(+)/Ca(2+) exchange currents to a greater extent in pulmonary vein cardiomyocytes, but decreased the delayed rectified potassium currents to a lesser extent. 5. Hypertonicity plays an important role in the electrical activity of pulmonary vein and atrial cardiomyocytes, which may have a potential role in the pathophysiology of atrial fibrillation.

Effect of hyperosmolality on beta-defensin gene expression by human corneal epithelial cells

PURPOSE

As human beta-defensins (hBD) are important antimicrobial peptides at epithelial surfaces, including the ocular surface, we tested the effect of hyperosmolar conditions on the expression of these peptides by human corneal epithelial cells (HCECs).

METHODS

Simian virus 40-transformed HCECs (n = 5) or primary cultured HCECs (n = 5) were treated with serum-free media or serum-free hyperosmolar (400-500 mOsm/kg) media for 24 hours or serum-free 500 mOsm/kg media for 12 to 48 hours. The effect of hyperosmolality on interleukin-1beta (IL-1beta)-induced hBD-2 expression was also tested. IL-6 expression was studied as a marker of IL-1beta function. Expression of hBD-1, -2, and -3 and IL-6 mRNA was detected by reverse transcription-polymerase chain reaction (RT-PCR). The levels of active IL-1beta (culture supernatants and cell lysates) and pro-IL-1beta (cell lysates) were detected by enzyme-linked immunosorbent assay.

RESULTS

HCECs constitutively expressed hBD-1 and -3 but not hBD-2. Hyperosmolar media had no effect on the basal expression of hBD-1 or -3 and did not induce the expression of hBD-2. Treatment with 500 mOsm/kg media for 24 hours decreased the ability of IL-1beta to upregulate hBD-2 and IL-6 expression. Active or pro-IL-1beta was not detected in any cell culture sample.

CONCLUSION

Our results suggest that the hyperosmolar environment observed in diseases such as dry eye does not alter defensin expression. However, a hyperosmolar environment may influence cytokine function in ocular surface cells and thus affect their response to injury and inflammation.

Acute hyperglycemia in uterine arteries from pregnant, but not non-pregnant mice, enhances endothelium-dependent relaxation

Poorly controlled diabetes in pregnancy, characterized by hypo- and hyperglycemia, is associated with adverse outcomes. We hypothesized that aberrant glucose levels affect vascular function in pregnancy. The effects of glucose concentration on constriction and endothelium-dependent relaxation in uterine arteries of normal C57BL/6 mice were examined. Ex-vivo arteries from 18 pregnant and 14 non-pregnant mice were mounted on a wire myograph, constricted with phenylephrine and relaxed with incremental doses of acetylcholine, in physiological saline solution containing 5 mmol/L glucose. Arteries were then exposed to solutions with 2, 5, 8 or 12 mmol/L glucose for 30 min and constriction/relaxation repeated. On altering glucose concentrations to 2, 8 or 12 mmol/L, maximal constriction was increased in arteries from pregnant but not from non-pregnant mice (paired t-test, p<0.05). Endothelium-dependent relaxation was enhanced at 12 mmol/L glucose in arteries from pregnant (two-way ANOVA, p<0.01), but not from non-pregnant mice. Endothelium-dependent relaxation in the uterine artery was pre-dominantly mediated by a non-nitric oxide/non-prostanoid mechanism, with a smaller contribution from nitric oxide, and no prostanoid-mediated relaxation. In summary, acute changes in glucose concentration alter both constriction and endothelium-dependent relaxation in uterine arteries of normal pregnant mice; these effects are unique to pregnancy.

Diabetic dyslipidemia and exercise affect coronary tone and differential regulation of conduit and microvessel K+ current

Spontaneous transient outward K(+) currents (STOCs) elicited by Ca(2+) sparks and steady-state K(+) currents modulate vascular reactivity, but effects of artery size, diabetic dyslipidemia, and exercise on these differentially regulated K(+) currents are unclear. We studied the conduit arteries and microvessels of male Yucatan swine assigned to one of three groups for 20 wk: control (C, n = 7), diabetic dyslipidemic (DD, n = 6), or treadmill-trained DD animals (DDX, n = 7). Circumflex artery blood flow velocity obtained with intracoronary Doppler and lumen diameters obtained by intravascular ultrasound enabled calculation of absolute coronary blood flow (CBF). Ca(2+) sparks were determined in pressurized microvessels, and perforated patch clamp assessed K(+) current in smooth muscle cells isolated from conduits and microvessels. Baseline CBF in DD was decreased versus C. In pressurized microvessels, Ca(2+) spark activity was significantly lower in DD versus C and DDX (P < 0.05 vs. DDX). STOCs were pronounced in microvessel (approximately 35 STOCs/min) in sharp contrast to conduit cells ( approximately 2 STOCs/min). STOCs were decreased by 86% in DD versus C and DDX in microvessels; in contrast, there was no difference in STOCs across groups in conduit cells. Steady-state K(+) current in microvessels was decreased in DD and DDX versus C; in contrast, steady-state K(+) current in conduit cells was decreased in DDX versus DD and C. We conclude that steady-state K(+) current and STOCs are differentially regulated in conduit versus microvessels in health and diabetic dyslipidemia. Exercise prevented diabetic dyslipidemia-induced decreases in baseline CBF, possibly via STOC-regulated basal microvascular tone.

Diabetes-induced activation of protein kinase C inhibits store-operated Ca2+ uptake in rat retinal microvascular smooth muscle

AIMS/HYPOTHESIS

To assess the effects of diabetes-induced activation of protein kinase C (PKC) on voltage-dependent and voltage-independent Ca2+ influx pathways in retinal microvascular smooth muscle cells.

METHODS

Cytosolic Ca2+ was estimated in freshly isolated rat retinal arterioles from streptozotocin-induced diabetic and non-diabetic rats using fura-2 microfluorimetry. Voltage-dependent Ca2+ influx was tested by measuring rises in [Ca2+]i with KCl (100 mmol/l) and store-operated Ca2+ influx was assessed by depleting [Ca2+]i stores with Ca2+ free medium containing 5 micromol/l cyclopiazonic acid over 10 min and subsequently measuring the rate of rise in Ca2+ on adding 2 mmol/l or 10 mmol/l Ca2+ solution.

RESULTS

Ca2+ entry through voltage-dependent L-type Ca2+ channels was unaffected by diabetes. In contrast, store-operated Ca2+ influx was attenuated. In microvessels from non-diabetic rats 20 mmol/l D-mannitol had no effect on store-operated Ca2+ influx. Diabetic rats injected daily with insulin had store-operated Ca2+ influx rates similar to non-diabetic control rats. The reduced Ca2+ entry in diabetic microvessels was reversed by 2-h exposure to 100 nmol/l staurosporine, a non-specific PKC antagonist and was mimicked in microvessels from non-diabetic rats by 10-min exposure to the PKC activator phorbol myristate acetate (100 nmol/l). The specific PKCbeta antagonist LY379196 (100 nmol/l) also reversed the poor Ca2+ influx although its action was less efficacious than staurosporine.

CONCLUSION/INTERPRETATION

These results show that store-operated Ca2+ influx is inhibited in retinal arterioles from rats having sustained increased blood glucose and that PKCbeta seems to play a role in mediating this effect.

Increased calcium buffering in coronary smooth muscle cells from diabetic dyslipidemic pigs

No studies exist concerning the ability of the plasma membrane Ca(2+) pump (PMCA), sarcoplasmic reticulum Ca(2+) pump (SERCA) and Na(+)-Ca(2+) exchanger (NCX) to regulate myoplasmic Ca(2+) (Ca(m)) in vascular smooth muscle cells from diabetic individuals with dyslipidemia. We tested the hypothesis that diabetic dyslipidemia would increase vascular smooth muscle cells to buffer Ca(m). Cells were isolated from the coronary artery of male Yucatan pigs treated for 20 weeks with: (1) a low fat diet (control group); (2) a high fat/cholesterol diet (F group); or (3) alloxan-induced diabetic pigs fed the high fat diet (DF group). The maximum Ca(m) response to a depolarizing 80 mM KCl (80 K) solution was evaluated in the absence and presence of thapsigargin (TSG; inhibits SERCA) and low Na (inhibits NCX). In response to 80 K alone, there was no difference in the Ca(m) response between groups. In the presence of TSG, the 80 K response decreased by 43% in the DF group; TSG did not affect the 80 K response in the control and F groups. When exposed to both TSG and low Na, the 80 K response also decreased by 55% in the DF group. This suggests increased Ca(m) buffering by the PMCA and/or mitochondria in the DF group when SERCA and NCX are inhibited. Compared to the control and F groups, low Na alone elicited a 50% lower Ca(m) amplitude in the DF group, which was reversed with TSG treatment; this suggests that SERCA activity is increased in DF pigs. Western blots also indicated a 7-fold increase in the approximately 115 kDa band density of an anti-SERCA2 antibody in DF compared to control pigs. This is the first report to demonstrate increased Ca(2+) buffering, specifically by SERCA, in vascular smooth muscle cells from diabetic individuals with dyslipidemia.

Reduced vasorelaxant effect of carbon monoxide in diabetes and the underlying mechanisms

Carbon monoxide (CO) is an endogenous gaseous factor that relaxes vascular tissues by acting on both the cGMP pathway and calcium-activated K+ (K(Ca)) channels. Whether the vascular effect of CO is altered in diabetes had been unknown. It was found that the CO-induced relaxation of tail artery tissues from streptozotocin-induced diabetic rats was significantly decreased as compared with that of nondiabetic control rats. The blockade of the cGMP pathway with ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one) completely abolished the CO-induced relaxation of diabetic tissues but only partially inhibited the CO effect in normal tissues. Single-channel conductance of K(Ca) channels in diabetic smooth muscle cells (SMCs) was not different from that of normal SMCs. However, the sensitivity of K(Ca) channels to CO in diabetic SMCs was significantly reduced. CO (10 micromol/l) induced an 81 +/- 24% increase in the mean open probability of single K(Ca) channels in normal SMCs but had no effect in diabetic SMCs. Longterm culture of normal vascular SMCs with 25 mmol/l glucose or 25 mmol/l 3-OMG (3-O-methylglucose) but not 25 mmol/l mannitol significantly reduced the sensitivity of K(Ca) channels to CO. On the other hand, the sensitivity of K(Ca) channels to CO was regained in diabetic SMCs that were cultured with 5 mmol/l glucose for a prolonged period. The decreased vasorelaxant effect of CO in diabetes represents a novel mechanism for the vascular complications of diabetes, which could be closely related to the glycation of K(Ca) channels in diabetic vascular SMCs.

Altered L-type Ca(2+) channel currents in vascular smooth muscle cells from experimental diabetic rats

Vascular complications of diabetes are associated with abnormal Ca(2+) handling by vascular smooth muscle cells (SMCs) in which the alteration in L-type voltage-dependent Ca(2+) channel (VDCC) currents may play an important role. In the present study, the characteristics of L-type VDCC currents in tail artery SMCs from streptozotocin-induced diabetic rats were examined. The densities, but not the voltage dependence, of L-type VDCC currents were reduced as diabetes progressed from 1 wk to 3 mo. The inhibitory effect of dibutyryl-cAMP on L-type VDCC currents was greater in diabetic SMCs than in age-matched control cells (P < 0.01). Both the stimulatory effect of BAY K 8644 and the inhibitory effect of nifedipine on L-type VDCC currents were significantly enhanced in diabetic cells. The diabetes-related abnormalities in L-type VDCC currents were mimicked by culturing SMCs with a high concentration of glucose. Our results suggest that the properties of L-type VDCC in diabetic vascular SMCs were significantly altered, partially related to the increased L-type VDCC sensitivity to cAMP and hyperglycemia.

Kinin B2 receptor-mediated contraction of tail arteries from normal or streptozotocin-induced diabetic rats

1. The vasoactive effects of bradykinin (BK) are mediated by different subtypes of kinin receptors, of which the expression varies among different tissues. In rat tail artery tissues, BK elicited a concentration-dependent vasoconstriction (EC50, 25.9+/-2.4nM; Emax, 0.39+/-0.01 g; n=16). This effect of BK was endothelium independent and indomethacin insensitive. The BK-induced contraction of tail artery tissues, however, depended on both membrane potential-sensitive extracellular Ca2+ entry and thapsigargin-sensitive intracellular Ca2+ release. 2. Kinin B1 receptor antagonist or agonist did not affect the basal tension or the BK-induced contraction of tail artery tissues in the absence or presence of endothelium (P>0.05). In contrast, the BK-induced vasoconstriction was inhibited by kinin B2 receptor antagonists. Pretreatment of vascular tissues with Hoe 140 (1 nM) significantly changed EC50 of the BK-induced vasoconstriction from 25.5+/-7.4 nM to 82.6+/-16.8nM (n=8, P<0.01) and Emax from 0.43+/-0.03g to 0.16+/-0.01 g (n=8, P<0.01). 3. In the tail artery tissues from streptozotocin-induced diabetic rats, the BK-elicited vasoconstriction was significantly reduced (EC50, 67.8+/-11 nM: Emax, 0.19+/-0.01 g) compared to their counterparts from normal rats. The decreased vasoconstrictive effects of BK on diabetic arteries were endothelium independent and indomethacin insensitive. 4. Our study demonstrated that the contraction of rat tail arteries induced by BK was mediated by B2 receptors located on vascular smooth muscles. The altered B2 receptor-mediated vascular activity may play an important role in the vascular complications of diabetes.