Stimulation of Na+-Ca2+ exchange in heart sarcolemma by insulin

The Cardiac Pacemaker Story-Fundamental Role of the Na+/Ca2+ Exchanger in Spontaneous Automaticity

The electrophysiological mechanism of the sinus node automaticity was previously considered exclusively regulated by the so-called "funny current". However, parallel investigations increasingly emphasized the importance of the Ca²⁺-homeostasis and Na⁺/Ca²⁺ exchanger (NCX). Recently, increasing experimental evidence, as well as insight through mechanistic in silico modeling demonstrates the crucial role of the exchanger in sinus node pacemaking. NCX had a key role in the exciting story of discovery of sinus node pacemaking mechanisms, which recently settled with a consensus on the coupled-clock mechanism after decades of debate. This review focuses on the role of the Na⁺/Ca²⁺ exchanger from the early results and concepts to recent advances and attempts to give a balanced summary of the characteristics of the local, spontaneous, and rhythmic Ca²⁺ releases, the molecular control of the NCX and its role in the fight-or-flight response. Transgenic animal models and pharmacological manipulation of intracellular Ca²⁺ concentration and/or NCX demonstrate the pivotal function of the exchanger in sinus node automaticity. We also highlight where specific hypotheses regarding NCX function have been derived from computational modeling and require experimental validation. Nonselectivity of NCX inhibitors and the complex interplay of processes involved in Ca²⁺ handling render the design and interpretation of these experiments challenging.

Insulin suppresses IKs (KCNQ1/KCNE1) currents, which require β-subunit KCNE1

Abnormal QT prolongation in diabetic patients has become a clinical problem because it increases the risk of lethal ventricular arrhythmia. In an animal model of type 1 diabetes mellitus, several ion currents, including the slowly activating delayed rectifier potassium current (IKs), are altered. The IKs channel is composed of KCNQ1 and KCNE1 subunits, whose genetic mutations are well known to cause long QT syndrome. Although insulin is known to affect many physiological and pathophysiological events in the heart, acute effects of insulin on cardiac ion channels are poorly understood at present. This study was designed to investigate direct electrophysiological effects of insulin on IKs (KCNQ1/KCNE1) currents. KCNQ1 and KCNE1 were co-expressed in Xenopus oocytes, and whole cell currents were measured by a two-microelectrode voltage-clamp method. Acute application of insulin suppressed the KCNQ1/KCNE1 currents and phosphorylated Akt and extracellular signal-regulated kinase (ERK), the two major downstream effectors, in a concentration-dependent manner. Wortmannin (10(-6) M), a phosphoinositide 3-kinase (PI3K) inhibitor, attenuated the suppression of the currents and phosphorylation of Akt by insulin, whereas U0126 (10(-5) M), a mitogen-activated protein kinase kinase (MEK) inhibitor, had no effect on insulin-induced suppression of the currents. In addition, insulin had little effect on KCNQ1 currents without KCNE1, which indicated an essential role of KCNE1 in the acute suppressive effects of insulin. Mutagenesis studies revealed amino acid residues 111-118 within the distal third C-terminus of KCNE1 as an important region. Insulin has direct electrophysiological effects on IKs currents, which may affect cardiac excitability.

Neurounina-1, a novel compound that increases Na+/Ca2+ exchanger activity, effectively protects against stroke damage

Previous studies have demonstrated that the knockdown or knockout of the three Na(+)/Ca(2+) exchanger (NCX) isoforms, NCX1, NCX2, and NCX3, worsens ischemic brain damage. This suggests that the activation of these antiporters exerts a neuroprotective action against stroke damage. However, drugs able to increase the activity of NCXs are not yet available. We have here succeeded in synthesizing a new compound, named neurounina-1 (7-nitro-5-phenyl-1-(pyrrolidin-1-ylmethyl)-1H-benzo[e][1,4]diazepin-2(3H)-one), provided with an high lipophilicity index and able to increase NCX activity. Ca(2+) radiotracer, Fura-2 microfluorimetry, and patch-clamp techniques revealed that neurounina-1 stimulated NCX1 and NCX2 activities with an EC(50) in the picomolar to low nanomolar range, whereas it did not affect NCX3 activity. Furthermore, by using chimera strategy and site-directed mutagenesis, three specific molecular determinants of NCX1 responsible for neurounina-1 activity were identified in the α-repeats. Interestingly, NCX3 became responsive to neurounina-1 when both α-repeats were replaced with the corresponding regions of NCX1. In vitro studies showed that 10 nM neurounina-1 reduced cell death of primary cortical neurons exposed to oxygen-glucose deprivation followed by reoxygenation. Moreover, in vitro, neurounina-1 also reduced γ-aminobutyric acid (GABA) release, enhanced GABA(A) currents, and inhibited both glutamate release and N-methyl-d-aspartate receptors. More important, neurounina-1 proved to have a wide therapeutic window in vivo. Indeed, when administered at doses of 0.003 to 30 μg/kg i.p., it was able to reduce the infarct volume of mice subjected to transient middle cerebral artery occlusion even up to 3 to 5 hours after stroke onset. Collectively, the present study shows that neurounina-1 exerts a remarkable neuroprotective effect during stroke and increases NCX1 and NCX2 activities.

Involvement of L-type calcium channel and SERCA2a in myocardial dysfunction induced by obesity

Obesity has been shown to impair myocardial performance. Nevertheless, the mechanisms underlying the participation of calcium (Ca(2+) ) handling on cardiac dysfunction in obesity models remain unknown. L-type Ca(2+) channels and sarcoplasmic reticulum (SR) Ca(2+) -ATPase (SERCA2a), may contribute to the cardiac dysfunction induced by obesity. The purpose of this study was to investigate whether myocardial dysfunction in obese rats is related to decreased activity and/or expression of L-type Ca(2+) channels and SERCA2a. Male 30-day-old Wistar rats were fed standard (C) and alternately four palatable high-fat diets (Ob) for 15 weeks. Obesity was determined by adiposity index and comorbidities were evaluated. Myocardial function was evaluated in isolated left ventricle papillary muscles under basal conditions and after inotropic and lusitropic maneuvers. L-type Ca(2+) channels and SERCA2a activity were determined using specific blockers, while changes in the amount of channels were evaluated by Western blot analysis. Phospholamban (PLB) protein expression and the SERCA2a/PLB ratio were also determined. Compared with C rats, the Ob rats had increased body fat, adiposity index and several comorbidities. The Ob muscles developed similar baseline data, but myocardial responsiveness to post-rest contraction stimulus and increased extracellular Ca(2+) was compromised. The diltiazem promoted higher inhibition on developed tension in obese rats. In addition, there were no changes in the L-type Ca(2+) channel protein content and SERCA2a behavior (activity and expression). In conclusion, the myocardial dysfunction caused by obesity is related to L-type Ca(2+) channel activity impairment without significant changes in SERCA2a expression and function as well as L-type Ca(2+) protein levels.

Insulin effects on cardiac Na+/Ca2+ exchanger activity: role of the cytoplasmic regulatory loop

Insulin can alter myocardial contractility, in part through an effect on the cardiac sarcolemmal Na(+)/Ca(2+) exchanger (NCX), but little is known about its mechanism of action. The large cytoplasmic domain (f-loop) of NCX is required for regulation by various intracellular factors, and we have shown previously that residues 562-679 are determinants of NCX inhibition by exchanger inhibitory peptide (XIP). Here we show that the same f-loop deletion eliminates the enhancement of NCX current by insulin, and we examine the signal pathways involved in the insulin response. NCX current (I(NCX)) was measured in freshly isolated or cultured (up to 48 h) adult guinea pig myocytes and in myocytes expressing canine NCX1.1 with the 562-679 f-loop deletion (NCX-(Delta562-679)) via adenoviral gene transfer. I(NCX) was recorded by whole-cell patch clamp as the Ni(2+)-sensitive current at 37 degrees C with intracellular Ca(2+) buffered. Insulin (1 microm) increased I(NCX) (at +80 mV) by 110 and 83% in fresh and cultured myocytes, respectively, whereas in myocytes expressing NCX-(Delta562-679) the response was eliminated (with 100 microm XIP included to suppress any native guinea pig I(NCX)). The insulin effect on I(NCX) was not inhibited by wortmannin, a nitric-oxide synthase inhibitor, or disruption of caveolae but was blocked by chelerythrine, implicating protein kinase C, but not phosphatidylinositol-3-kinase, in the mechanism. The insulin effect was also not additive with phosphatidylinositol-4,5-bisphosphate-induced activation of I(NCX). The finding that the 562-670 f-loop domain is implicated in both XIP and receptor-mediated modulation of NCX highlights its important role in acute physiological or pathophysiological regulation of Ca(2+) balance in the heart.

Insulin effects on myocardial function and bioenergetics in L-bupivacaine toxicity in the isolated rat heart

BACKGROUND AND OBJECTIVES

A positive effect of insulin-glucose-potassium infusion in severe bupivacaine-induced cardiovascular collapse has been described in vivo. It has been speculated that an antagonistic influence of insulin on sodium channel inhibition, transient outward potassium current, calcium-dependent adenosine triphosphatase or even improved myocardial energetics may be responsible for this effect. Using an isolated heart model, we therefore sought to further elucidate insulin effects in l-bupivacaine-induced myocardial depression.

METHODS

An isolated rat heart constant-pressure perfused, non-recirculating Langendorff preparation was used. Hearts were exposed to l-bupivacaine 5 microg mL(-1) and insulin 10 mIU mL(-1). Heart rate, systolic pressure, the first derivative of left ventricular pressure (+dP/dt), coronary flow, double product, PR and QRS intervals were recorded. Hearts were freeze-clamped and high-performance liquid chromatography measurement of the total adenine nucleotide pool was performed.

RESULTS

l-Bupivacaine led to a significant decrease in heart rate, +dP/dt, systolic pressure, coronary flow and double product, and to an increase in PR and QRS. Insulin exerted a positive inotropic effect, significantly augmenting +dP/dt and systolic pressure in both l-bupivacaine-treated and control hearts. Heart rate, coronary flow, total adenine nucleotides, PR and QRS were not significantly changed by the insulin intervention.

CONCLUSION

Insulin did not have a significant effect on total adenine nucleotides in controls and in l-bupivacaine-treated hearts. However, it does exert a positive inotropic action in bupivacaine-induced myocardial depression. We conclude that the positive effect of insulin application lies in positive inotropic action and not in changes in total adenine nucleotides.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Insulin inhibits coronary endothelial cell calcium entry and coronary artery relaxation

Hyperinsulinemia is closely related to coronary artery disease. Endothelial cells are important for the control of vascular tone, and dysfunction of endothelial cells has been implicated in coronary artery disease. The direct effects of insulin on coronary endothelial cells are nonetheless unknown. In this study, the acute effects of high-dose insulin were investigated on agonist-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) in porcine coronary endothelial cells and coronary relaxation. Bradykinin (10 n M ) and cyclopiazonic acid (100 microM), an inhibitor of the endoplasmic reticulum Ca(2+)-ATPase, provoked large increases in [Ca(2+)](i) in coronary endothelial cells. This increase was dose-dependently inhibited by a 10-min preincubation with high doses of insulin (10, 30, 100 mU/ml). Under Ca(2+)-free conditions, bradykinin and cyclopiazonic acid provoked transient, small increases in [Ca(2+)](i). These increases were not affected by pretreatment with insulin (100 mU/ml). Bradykinin (1, 10, 100, 1,000 n M ) and cyclopiazonic acid (10 microM) significantly relaxed porcine coronary artery rings precontracted with histamine (1 microM). The vasodilator effects of bradykinin and cyclopiazonic acid were dose-dependently inhibited by insulin. These acute effects were not observed at physiologic concentrations. Our data indicate that high-dose insulin inhibits agonist-induced Ca(2+) response in coronary endothelial cells and attenuates agonist-induced coronary vasodilatation. The study suggests that hyperinsulinemia might be associated with coronary artery disease via derangement of endothelial Ca(2+)-dependent functions.

Endogenous digoxin-like immunoactivity and diabetes mellitus: facts and hypotheses

Substances with digoxin- and ouabain-like immunoactivity (DLIA) are specific inhibitors of Na(+)-K(+)-ATPase which increase the total amount of intracellular stored calcium (Ca2+i). In diabetic patients, DLIA levels have been reported to be increased. Although this increase is probably secondary to sodium retention and volume expansion (included in diabetic subjects by hyperinsulinemia and/or diabetic nephropathy), the question arises of whether it has pathophysiological consequences: namely, whether substances with DLIA, via their effect on Na(+)-K(+)-ATPase activity and Ca2+i stores, could in diabetic subjects facilitate development of hypertension and/or modulate insulin sensitivity or insulin secretion. Clinical findings of correlations of DLIA to blood pressure, insulin levels and to degree of insulin resistance, together with experimental findings of decreased Na(+)-K(+)-ATPase activity, increased Ca2+i and decreased Mg2+i in both diabetic and hypertensive subjects, support these hypotheses. However, the issue of whether or not these relations are causative and whether or not defects in intracellular milieu are primary or secondary to non-insulin-dependent diabetes mellitus has not been resolved yet. Moreover, pathogenesis of both diabetes mellitus and hypertension is multifactorial and includes many other factors. Therefore, further efforts should be made to elucidate the exact role of substances with DLIA in diabetes mellitus.

Mechanisms of inotropic responses of the isolated rat hearts to vanadate

In view of the invariable development of insulin resistance in different types of cardiovascular diseases, considerable attention has been focused on vanadate because of its ability to exert insulin-like effects in the body. Since vanadate, like insulin, has been shown to exert a beneficial effect in diabetic cardiomyopathy, this study was undertaken to examine the mechanisms of its action on the heart. Vanadate, at 5-10 microM concentrations, produced a positive inotropic effect in the isolated perfused rat heart, whereas at higher concentrations (20 microM), it decreased the contractile force development. The positive inotropic effect of 10 microM vanadate was not affected by the pretreatment of animals with reserpine as well as the presence of propranolol or phenoxybenzamine in the perfusion medium. The increase in contractile force development due to vanadate at low (0.3-0.6 mM) concentrations of Ca2+ was markedly augmented, but this agent produced a negative inotropic action at high concentrations of Ca2+ (2.0-3.0 mM). Preperfusion of hearts with verapamil enhanced the positive inotropic effect of vanadate whereas hearts preperfused with ouabain, low sodium or amiloride showed negative inotropic effects of vanadate. Vanadate was found to inhibit sarcoplasmic reticular Ca(2+)-pump and sarcolemmal Ca(2+)-pump as well as Na(+)-K(+)-ATPase activities but the sarcolemmal effects were evident at lower concentrations in comparison to that on the sarcoplasmic reticulum. The actions of vanadate on membrane Ca2+ transport and ATPase systems were specific since this agent exerted no effect on sarcolemmal Na(+)-Ca2+ exchange or myofibrillar ATPase activities. In isolated cardiomyocytes suspended in buffer containing 0.5 or 1.0 mM Ca2+, vanadate increased the intracellular concentration of Ca2+; this increase in intracellular Ca2+ was more pronounced at 0.5 mM Ca2+. These results indicate that increased intracellular concentration of Ca2+ due to inhibition of sarcolemmal Na(+)-K(+)-ATPase and sarcolemmal Ca(2+)-pump may be the primary mechanism of the positive inotropic action of vanadate in the heart. It is suggested that vanadate may serve as an inotropic agent and that this mechanism may contribute towards its beneficial effects on cardiac dysfunction in different cardiovascular diseases.

Decreased activity of (Ca2+ + Mg2+)-adenosine triphosphatase (ATPase) and a hormone-specific defect in insulin regulation of ATPase in kidney basolateral membranes from obese fa/fa rats

The plasma membrane enzyme (Ca2+ + Mg2+)-adenosine triphosphatase (ATPase) is hormonally regulated and may participate in Ca2+ signaling by removing excess Ca2+ from the cell. Therefore, observations of a hormone-specific loss of insulin stimulation of ATPase in kidney membranes from non-insulin-dependent diabetic (NIDDM) rats may reflect their insulin-resistant state. Consequently, to evaluate whether additional insulin-resistant conditions are associated with impaired function of ATPase and with loss of regulation of the enzyme by insulin, studies were extended to investigate (Ca2+ + Mg2+)-ATPase activities and hormonal regulation of the enzyme in kidney basolateral membranes from obese and lean Zucker rats. (Ca2+ + Mg2+)-ATPase activity was lower in membranes from obese rats compared with lean rats. Maximal velocity (Vmax) of the enzyme activity was 29.2 +/- 2.6 nmol Pi/mg/min in obese rats versus 57.2 +/- 6.5 in lean rats (P < .05). However, the affinity of the enzyme for Ca2+ was similar in obese and lean rats (Km Ca2+, 0.23 +/- 0.025 v 0.23 +/- 0.032 mumol/L Ca2+). Also, the Km for ATP of the enzyme was similar in membranes from obese and lean rats. Insulin, parathyroid hormone (PTH), and cyclic adenosine monophosphate (cAMP) stimulated the ATPase activity in membranes from lean rats in a dose-dependent manner (15% to 28%). Also, the protein kinase C (PKC) stimulator 12-O-tetradecanoyl phorbol-13-acetate (TPA) increased the ATPase activity in membranes from lean rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormone-specific defect in insulin regulation of (Ca2+ + Mg2+)-adenosine triphosphatase activity in kidney membranes from streptozocin non-insulin-dependent diabetic rats

The plasma membrane enzyme (Ca2+ + Mg2+)-adenosine triphosphatase [(Ca2+ + Mg2+)-ATPase] is hormonally regulated, and may participate in Ca2+ signaling by removing excess Ca2+ from the cell. Insulin increases ATPase activity in kidney cortical basolateral membranes (BLM) from normal rats, but fails to do so in membranes from insulin-resistant non-insulin-dependent diabetic (NIDDM) rats. To investigate mechanisms of insulin regulation of ATPase and to evaluate whether the loss of this regulation in diabetes is hormone-specific and depends on blood glucose levels, (Ca2+ + Mg2+)-ATPase function and its hormonal regulation were studied in kidney BLM from rats with mild and severe NIDDM. Km values for ATP and Ca2+ affinity of the ATPase were similar in diabetic and control rats, but the maximal velocity (Vmax) of the enzyme was higher in diabetic groups. Insulin, the protein kinase C (PKC) stimulator 12-0-tetradecanoylphorbol 13-acetate (TPA), parathyroid hormone (PTH), and cyclic adenosine monophosphate (cAMP) all increased the ATPase activity in BLM from controls by increasing the enzyme's affinity for Ca2+. A protein kinase A (PKA) inhibitor (H8 in low concentrations) abolished cAMP and PTH effects, but not those of insulin, whereas the PKC inhibitors (sphingosine and high concentrations of H8) did abolish the effects of insulin. Stimulations of ATPase activity by insulin and by PTH and cAMP were additive. Insulin and TPA lost their stimulatory effects on ATPase in BLM from rats with either mild or severe NIDDM, but PTH and cAMP maintained their stimulatory effects in these membranes. The data show [1] (Ca2+ + Mg2+)-ATPase activity is increased in NIDDM, and a hormone-specific loss of insulin stimulation of ATPase occurs; (2) these defects are not dependent on the level of glycemia; and (3) the stimulatory effects of insulin on the ATPase may be mediated in part via PKC. We suggest that the hormone-specific defect in insulin regulation of ATPase seen in the NIDDM rats may contribute to their insulin resistance.

Insulin attenuates agonist-mediated calcium mobilization in cultured rat vascular smooth muscle cells

Insulin has been shown to attenuate pressor-induced vascular contraction, but the mechanism for this vasodilatory action is unknown. This study examines the effect of insulin on angiotensin II (ANG II)-induced increments in cytosolic calcium in cultured rat vascular smooth muscle cells (VSMC). 20-min incubations with insulin (10 microU/ml to 100 mU/ml) did not alter basal intracellular calcium concentration ([Ca2+]i), but inhibited the response to 100 nM ANG II in a dose-dependent manner (ANG II alone, 721 +/- 54 vs. ANG II + 100 mU/ml insulin, 315 +/- 35 nM, P < 0.01). A similar effect of insulin on ANG II action was observed in calcium poor buffer. Moreover, insulin did not effect calcium influx. ANG II receptor density and affinity were not affected by 24-h incubation with insulin. To further clarify the mechanisms of these observations, we measured ANG II-induced production of inositol 1,4,5-triphosphate (IP3), and IP3-releasable 45Ca. Insulin treatment did not alter ANG II-stimulated IP3 production. However, IP3-stimulated release of 45Ca in digitonin permeabilized cells was significantly reduced after 5-min incubations with 100 mU/ml insulin. Thapsigargin induced release of calcium stores was also blocked by insulin. Thus, insulin attenuates ANG II-stimulated [Ca2+]i primarily by altering IP3-releasable calcium stores. Insulin effects on ANG II-induced [Ca2+]i were mimicked by preincubation of VSMC with either sodium nitroprusside or 8-bromo-cGMP. As elevations in cGMP in vascular tissue lower [Ca2+]i, it is possible that insulin affects IP3 release of calcium by a cGMP-dependent mechanism that would contribute to its vasodilatory effects.

Characteristics and mechanisms of tachyphylaxis of cardiac contractile response to insulin

Although insulin is known to cause internalization of its own receptors, the physiological significance of this phenomenon is not clear. In the isolated rat heart we observed that the positive inotropic effect of 25 munits/ml insulin was completely abolished if the heart was preperfused with insulin for 10 min. This tachyphylactic response to insulin began to appear 3-4 min after starting preperfusion with insulin and was partially reversible after 30 min of washing. Preperfusion with insulin did not affect the action of vanadate, which has insulin-like effect on glucose transport, or the actions of the other positive inotropic agents, isoproterenol and ouabain. The presence of propranolol in the perfusion medium, unlike atenolol, phenoxybenzamine, guanethidine, verapamil or quinidine, modified the inotropic as well as tachyphylactic responses to insulin. The positive inotropic and tachyphylactic responses to insulin were not altered in hearts from reserpine-treated animals. Perfusion of heart with glucose-free solution abolished the tachyphylaxis due to insulin. Likewise, no tachyphylactic response to insulin was evident when iodoacetate, but not sodium fluoride, was added in medium containing glucose. These results suggest that ATP formed during glycolysis may play an important role in insulin-induced tachyphylaxis with respect to cardiac contractile activity.

Protective effect of increased glycolytic substrate against systolic and diastolic dysfunction and increased coronary resistance from prolonged global underperfusion and reperfusion in isolated rabbit hearts perfused with erythrocyte suspensions

Current therapy of myocardial infarction may include early reperfusion. We simulated myocardial perfusion conditions during evolving myocardial infarction in isolated, normothermic, isovolumic rabbit hearts perfused with buffer containing bovine red blood cells (hematocrit of 40%), and we assessed the effects of high levels of glucose and insulin as "therapy" during prolonged (150-minute) severe underperfusion and reperfusion. Protocol 1 consisted of underperfusion at a constant coronary perfusion pressure of 8 mm Hg. The control group (n = 8) received 5.5 mmol/l glucose and 15 microunits/ml insulin; the group treated with high levels of glucose and insulin (G + I) (n = 8) received 19.5 mmol/l glucose and 250 microunits/ml insulin during both underperfusion and reperfusion. Relative to the control group, the G + I group experienced 1) greater developed pressure during underperfusion and increased recovery during reperfusion, 2) preserved diastolic function during underperfusion and reperfusion, 3) lower coronary resistance and greater coronary flow during the underperfusion period, 4) increased glycolytic flux and preserved glycogen stores and high energy phosphate levels, and 5) less loss of myocyte enzymes (creatine kinase and alanine aminotransferase). In protocol 2, coronary flow was kept identical in control (n = 8) and G + I hearts (n = 8) during the underperfusion period, and left ventricular end-diastolic pressure was kept below 10 mm Hg in both groups to minimize subendocardial damage and vascular compression. In this protocol, the effect of the G + I intervention in the prevention of an increase in coronary resistance during the underperfusion period was distinguished from its myocellular metabolic effects; the high G + I substrate had protective effects on mechanical and metabolic function that were less marked than, but similar to, those in protocol 1, indicating that its mechanisms of protection during underperfusion affected both cardiac function and coronary resistance. We conclude that the G + I intervention, in clinically relevant concentrations, markedly protected severely underperfused myocardium for 150 minutes and may be a beneficial intervention in combination with reperfusion therapy in acute myocardial infarction.

Improvement of hypoperfusion with norepinephrine injury by ex vivo insulin in isolated diabetic rat hearts

Effects of insulin on contractile and energy metabolic dysfunctions during hypoperfusion (2 ml/min/g heart wt., 60 min) with 10(-6) M norepinephrine were studied in paced hearts isolated from streptozotocin-diabetic rats. Insulin (2 mU/min/g heart wt.) was infused 20 min before and during hypoperfusion (pre-treated group) or 30 min after the onset of hypoperfusion (post-treated group). Hearts in the non-treated group were hypoperfused without insulin and other hearts in the control group were not hypoperfused. In the non-treated group, resting contractile force (CF) and resting left ventricular pressure (LVP) were significantly elevated to maximum levels within 30 min after hypoperfusion and these elevations were restored in the pre-treated group but not in the post-treated group. Developed CF was depressed in the non-treated group and improved significantly in the pretreated group but not in the post-treated group. Developed LVP was depressed in the non-treated group, and depression was slightly larger in the pre-treated group. In the non-treated group, ATP and creatine phosphate contents in the left ventricle significantly decreased. Decreases in ATP and creatine phosphate contents in the inner layer were partially restored in the pre-treated group but not in the post-treated group. Lactate significantly increased in the non-treated group and increased even further in the insulin treated groups. These results indicate that contractile dysfunction during hypoperfusion with norepinephrine is improved by pre-treated insulin, as is partial recovery of energy metabolism.

Effects of food restriction and insulin treatment on (Ca2+ + Mg2+)-ATPase response to insulin in kidney basolateral membranes of noninsulin-dependent diabetic rats

Insulin increases (Ca2+ + Mg2+)-ATPase activity in cell membranes of normal rats but fails to do so in membranes of non-insulin-dependent diabetic (NIDD) rats. The loss of regulatory effect of the hormone on the enzyme might contribute to the insulin resistance observed in the NIDD animals. To further test this hypothesis, the effects of insulin treatment and acute food restriction on the ability of insulin to regulate the ATPase activity in kidney basolateral membranes (BLM) of NIDD rats were studied. Although insulin levels in NIDD and control rats were similar, plasma glucose was higher in the NIDD rats (18.3 +/- 1.5 v 19.3 +/- 1.7 microU/mL and 236 +/- 32 v 145 +/- 3 mg/dL, respectively). Insulin treatment (2 U/100 g), which increased plasma insulin in the NIDD rats (47.8 +/- 11.5 microU/mL; P less than .05), did not decrease their glucose (221 +/- 25 mg/dL). Higher insulin dose (4 U/100 g) decreased glucose level in the NIDD rats (73 +/- 3 mg/dL; P less than .001) but increased their plasma insulin 10-fold (202.5 +/- 52.5 microU/mL). Acute food restriction decreased glucose levels in the NIDD rats to levels seen in controls (135 +/- 3 mg/dL), while their insulin decreased by half (8.5 +/- 1.0 microU/mL; P less than .05). Basal (Ca2+ + Mg2+)-ATPase activity in BLM of all diabetic rats was higher than in controls (P less than .05). None of the treatments reversed this defect.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of cellular calcium metabolism in abnormal glucose metabolism and diabetic hypertension

The prevalence of hypertension in patients with non-insulin-dependent diabetes mellitus (NIDDM) is considerably higher than in the non-diabetic population. Insulin resistance may contribute to this increased prevalence. Abnormal cellular calcium (Ca2+) homeostasis may link insulin resistance and high blood pressure in patients with NIDDM. Observations of abnormal cellular Ca2+ homeostasis in animal models of NIDDM and obesity as well as in diabetic patients are consistent with this hypothesis. Abnormalities in cellular Ca2+ homeostasis are also found in hypertensive animals and humans. Alterations in cell membrane phospholipid content and distribution may be the primary cause of abnormal plasma membrane Ca2+ fluxes in patients with NIDDM and hypertension.

Abnormal cell calcium concentrations in cultured bone cells obtained from femurs of obese and noninsulin-dependent diabetic rats

Cytoplasmic free calcium concentration [Ca2+]i was quantified in cultured bone cells with osteoblastic characteristics. The cells were obtained from femurs of obese (fa/fa) Wistar-Kyoto rats, from nonobese, noninsulin-dependent diabetic (NIDD) Sprague Dawley rats, and from their appropriate controls. [Ca2+]i was also determined in bone cells obtained from in vivo insulin-treated NIDD rats. Obese (Wistar Kyoto) rats had increased body weight (313 +/- 13 vs. 249 +/- 4 g; P less than 0.01), decreased femur weights (0.68 +/- 0.05 vs. 0.89 +/- 0.05 g; P less than 0.05), similar glucose levels (148 +/- 5 vs. 139 +/- 3 mg/dl), and higher plasma insulin levels (6.0 +/- 0.5 vs. 0.7 +/- 0.1 ng/ml; P less than 0.01) when compared with their nonobese [(fa/+); (+/+)] littermates. Nonobese, NIDD rats, compared with their appropriate controls (nondiabetic Sprague Dawley rats) had higher plasma glucose levels (235 +/- 32 vs. 145 +/- 3 mg/dl; P less than 0.01) but their plasma insulins, body weights, and femur weights were similar to controls (0.7 +/- 0.1 vs. 0.6 +/- 0.1 ng/ml; 302 +/- 4 vs. 318 +/- 14 g; 0.97 +/- 0.4 vs. 0.98 +/- 0.04 g, respectively). Long-term (4 weeks) daily insulin treatment (2 u/100 g) of the NIDD rats increased their plasma insulin (1.9 ng/ml; P less than 0.05) and body weight (369 +/- 13 g; P less than 0.05) but did not change their plasma glucose levels (225 +/- 5 mg/dl), or femur weights (0.98 +/- 0.4 g).(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension and diabetes

Diabetes mellitus and hypertension are both common diseases, especially with an increasingly aged population. Hypertension accelerates the development of diabetic retinopathy, nephropathy, and peripheral vascular disease in the diabetic patient. Diabetes represents a type of premature aging and hypertension in the diabetic patient is characterized by many of the same pathophysiologic properties seen in the elderly hypertensive patient.