Ageing-related cardiomyocyte functional decline is sex and angiotensin II dependent.

Clinically, heart failure is an age-dependent pathological phenomenon and displays sex-specific characteristics. The renin-angiotensin system mediates cardiac pathology in heart failure. This study investigated the sexually dimorphic functional effects of ageing combined with angiotensin II (AngII) on cardiac muscle cell function, twitch and Ca(2+)-handling characteristics of isolated cardiomyocytes from young (~13 weeks) and aged (~87 weeks) adult wild type (WT) and AngII-transgenic (TG) mice. We hypothesised that AngII-induced contractile impairment would be exacerbated in aged female cardiomyocytes and linked to Ca(2+)-handling disturbances. AngII-induced cardiomyocyte hypertrophy was evident in young adult mice of both sexes and accentuated by age (aged adult ~21-23 % increases in cell length relative to WT). In female AngII-TG mice, ageing was associated with suppressed cardiomyocyte contractility (% shortening, maximum rate of shortening, maximum rate of relaxation). This was associated with delayed cytosolic Ca(2+) removal during twitch relaxation (Tau ~20 % increase relative to young adult female WT), and myofilament responsiveness to Ca(2+) was maintained. In contrast, aged AngII-TG male cardiomyocytes exhibited peak shortening equivalent to young TG; yet, myofilament Ca(2+) responsiveness was profoundly reduced with ageing. Increased pro-arrhythmogenic spontaneous activity was evident with age and cardiac AngII overexpression in male mice (42-55 % of myocytes) but relatively suppressed in female aged transgenic mice. Female myocytes with elevated AngII appear more susceptible to an age-related contractile deficit, whereas male AngII-TG myocytes preserve contractile function with age but exhibit desensitisation of myofilaments to Ca(2+) and a heightened vulnerability to arrhythmic activity. These findings support the contention that sex-specific therapies are required for the treatment of age-progressive heart failure.

Fructose diet treatment in mice induces fundamental disturbance of cardiomyocyte Ca2+ handling and myofilament responsiveness.

High fructose intake has been linked to insulin resistance and cardiac pathology. Dietary fructose-induced myocardial signaling and morphological alterations have been described, but whether cardiomyocyte function is influenced by chronic high fructose intake is yet to be elucidated. The goal of this study was to evaluate the cardiomyocyte excitation-contraction coupling effects of high dietary fructose and determine the capacity for murine cardiomyocyte fructose transport. Male C57Bl/6J mice were fed a high fructose diet for 12 wk. Fructose- and control-fed mouse cardiomyocytes were isolated and loaded with the fura 2 Ca(2+) fluorescent dye for analysis of twitch and Ca(2+) transient characteristics (4 Hz stimulation, 37°C, 2 mM Ca(2+)). Myocardial Ca(2+)-handling protein expression was determined by Western blot. Gene expression of the fructose-specific transporter, GLUT5, in adult mouse cardiomyocytes was detected by real-time and conventional RT-PCR techniques. Diastolic Ca(2+) and Ca(2+) transient amplitude were decreased in isolated cardiomyocytes from fructose-fed mice relative to control (16 and 42%, respectively), coincident with an increase in the time constant of Ca(2+) transient decay (24%). Dietary fructose increased the myofilament response to Ca(2+) (as evidenced by a left shift in the shortening-Ca(2+) phase loop). Protein expression of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a), phosphorylated (P) phospholamban (Ser(16)), and P-phospholamban (Thr(17)) was reduced, and protein phosphatase 2A expression increased, in fructose-fed mouse hearts. Hypertension and cardiac hypertrophy were not evident. These findings demonstrate that fructose diet-associated myocardial insulin resistance induces profound disturbance of cardiomyocyte Ca(2+) handling and responsiveness in the absence of altered systemic loading conditions.

Fructose modulates cardiomyocyte excitation-contraction coupling and Ca²âº handling in vitro.

BACKGROUND: High dietary fructose has structural and metabolic cardiac impact, but the potential for fructose to exert direct myocardial action is uncertain. Cardiomyocyte functional responsiveness to fructose, and capacity to transport fructose has not been previously demonstrated. OBJECTIVE: The aim of the present study was to seek evidence of fructose-induced modulation of cardiomyocyte excitation-contraction coupling in an acute, in vitro setting. METHODS AND RESULTS: The functional effects of fructose on isolated adult rat cardiomyocyte contractility and Ca²âº handling were evaluated under physiological conditions (37°C, 2 mM Ca²âº, HEPES buffer, 4 Hz stimulation) using video edge detection and microfluorimetry (Fura2) methods. Compared with control glucose (11 mM) superfusate, 2-deoxyglucose (2 DG, 11 mM) substitution prolonged both the contraction and relaxation phases of the twitch (by 16 and 36% respectively, p<0.05) and this effect was completely abrogated with fructose supplementation (11 mM). Similarly, fructose prevented the Ca²âº transient delay induced by exposure to 2 DG (time to peak Ca²âº transient: 2 DG: 29.0±2.1 ms vs. glucose: 23.6±1.1 ms vs. fructose +2 DG: 23.7±1.0 ms; p<0.05). The presence of the fructose transporter, GLUT5 (Slc2a5) was demonstrated in ventricular cardiomyocytes using real time RT-PCR and this was confirmed by conventional RT-PCR. CONCLUSION: This is the first demonstration of an acute influence of fructose on cardiomyocyte excitation-contraction coupling. The findings indicate cardiomyocyte capacity to transport and functionally utilize exogenously supplied fructose. This study provides the impetus for future research directed towards characterizing myocardial fructose metabolism and understanding how long term high fructose intake may contribute to modulating cardiac function.

Exploiting cGMP-based therapies for the prevention of left ventricular hypertrophy: NO* and beyond.

Left ventricular hypertrophy (LVH), an increased left ventricular (LV) mass, is common to many cardiovascular disorders, initially developing as an adaptive response to maintain myocardial function. In the longer term, this LV remodelling becomes maladaptive, with progressive decline in LV contractility and diastolic function. Indeed LVH is recognised as an important blood-pressure independent predictor of cardiovascular morbidity and mortality. The clinical efficacy of current treatments for LVH is reduced, however, by their tendency to slow disease progression rather than induce its reversal, and thus the development of new therapies for LVH is paramount. The signalling molecule cyclic guanosine-3',5'-monophosphate (cGMP), well-recognised for its role in regulating vascular tone, is now being increasingly identified as an important anti-hypertrophic mediator. This review is focused on the various means by which cGMP can be stimulated in the heart, such as via the natriuretic peptides, to exert anti-hypertrophic actions. In particular we address the limitations of traditional nitric oxide (NO*) donors in the face of the potential therapeutic advantages offered by novel alternatives; NO* siblings, ligands of the cGMP-generating enzymes, soluble (sGC) and particulate guanylyl cyclases (pGC), and phosphodiesterase inhibitors. Further impact of cGMP within the cardiovascular system is also discussed with a view to representing cGMP-based therapies as innovative pharmacotherapy, alone or concurrent with standard care, for the management of LVH.

Sex differences in cardiac muscle responsiveness to Ca2+ and L-type Ca2+ channel modulation.

This study investigated sex-related differences in rat papillary muscle force generation in response to altered extracellular [Ca2+] ([Ca2+](o), 0.2 to 5.0 mM) and to L-type Ca2+ channel modulators (nifedipine and Bay K8644). At all [Ca2+]o examined, contractile force was significantly greater in male than female papillary muscles. The [Ca2+]o required for 50% maximum force was significantly lower in male [0.34+/-0.06 mM] than female [0.61+/-0.10 mM] papillary muscles. Nifedipine decreased contractile force in papillary muscles of both sexes in a concentration-dependent manner, but the extent of the contractile depression was more marked in male papillary muscles at all nifedipine concentrations examined. BayK 8644 produced a concentration-dependent increase in contractile force in male papillary muscles but notably, not in female papillary muscles. These findings show that sex differences in myocardial mechanical function are associated with sex-specific modulation of L-type Ca2+ channel responsiveness. Thus, the L-type Ca2+ channel could represent an important cellular locus from which sex-based differences in myocardial excitation-contraction coupling arise.

Energy cost of contraction in rat urinary bladder smooth muscle during anoxia.

1. The aim of the present study was to investigate the effects of hypoxia on energy metabolism and contraction of rat urinary bladder smooth muscle, thereby gaining insight into the capacity of this smooth muscle to maintain contractile function when rendered hypoxic. 2. Isometric force, oxygen consumption, lactate production, heat production and unloaded shortening velocity were measured in isolated muscle strips under both aerobic and anaerobic conditions. Muscle strips were bathed in physiological saline solution with the anaerobic condition being created by replacing the oxygen bubbling the solution with nitrogen. 3. During contraction under anaerobic conditions, the rate of lactate production was increased 2.5-fold above that observed under aerobic conditions. This, however, only provided for a rate of ATP production of approximately 30% of that measured under aerobic conditions. Despite this, force maintenance was only slightly depressed, indicating that the metabolic cost of contraction was reduced in hypoxia. In support of this, the rate of heat production during contractions in anoxia was only approximately half of that under aerobic conditions, whereas, again, force was only slightly lower. Unloaded shortening velocity was significantly lower in anoxia, suggesting a slower cross-bridge turnover rate. 4. The results indicate that the economy of force maintenance is increased in bladder smooth muscle under hypoxic conditions and that this is due, at least in part, to a reduced rate of cross-bridge cycling. This may help to preserve bladder contractile function during periods of ischaemia that may be associated with bladder filling and emptying.

Effects of ovariectomy and 17 beta-oestradiol replacement on [Ca2+]i in female rat cardiac myocytes.

1. The present study investigated the effects of ovariectomy (OVX) and 17beta-oestradiol replacement on [Ca2+]i in rat freshly isolated cardiac myocytes. 2. Myocytes were isolated from the hearts of sham, OVX and OVX + 17beta-oestradiol-replaced female rats by enzymatic digestion with collagenase. Changes in [Ca2+]i in response to varied extracellular [Ca2+] were measured using the Ca2+-sensitive dye fura-2, with the contractile responses of each cell measured as cell shortening. 3. Increasing extracellular [Ca2+] resulted in increased [Ca2+]i in all three groups. Peak [Ca2+]i and the amplitude of the Ca2+ transient were significantly greater (P < 0.01) in cells from OVX animals compared with cells from both sham and 17beta-oestradiol-replaced OVX animals. 4. The time-course of decay of the Ca2+ transient was significantly faster (P < 0.02) in OVX cells compared with both sham and 17beta-oestradiol-replaced cells. In addition, time to 50% relaxation was significantly faster (P < 0.04) and extent of shortening significantly greater (P < 0.01) in OVX cells than in either sham or 17beta-oestradiol cells. 5. These data demonstrate clear differences in peak [Ca2+]i and the amplitude of the Ca2+ transient between OVX female rat cardiac myocytes compared with intact and 17beta-oestradiol-replaced OVX female rat cardiac myocytes. This suggests that oestrogen may play a long-term role in limiting Ca2+ entry into the cardiac myocyte.