Irregular rhythm adversely influences calcium handling in ventricular myocardium: implications for the interaction between heart failure and atrial fibrillation.

BACKGROUND: Despite adequate rate control, the combination of atrial fibrillation with heart failure (HF) has been shown, in a number of studies, to hasten HF progression. In this context, we aimed to test the hypothesis that an irregular ventricular rhythm causes an alteration in ventricular cardiomyocyte excitation-contraction coupling which contributes to the progression of HF. METHODS AND RESULTS: We investigated the effects of electrical field stimulation (average frequency 2 Hz) in an irregular versus regular drive train pattern on the expression of calcium-handling genes and proteins in rat ventricular myocytes. The effect of rhythm on intracellular calcium transients was examined using Fura-2AM fluorescence spectroscopy. In conjunction, calcium-handling protein expression was examined in left ventricular samples obtained from end-stage HF patients, in patients with either persistent atrial fibrillation or sinus rhythm. Compared with regularly paced ventricular cardiomyocytes, in cells paced irregularly for 24 hours, there was a significant reduction in the expression of sarcoplasmic reticulum calcium (Ca(2+)) ATPase together with reduced serine-16 phosphorylation of phospholamban. These findings were accompanied by a 59% reduction (P<0.01) in the peak Ca2+ transient in irregulary paced myocytes compared with those with regular pacing. Consistent with these observations, we observed a 54% (P<0.05) decrease in sarcoplasmic reticulum Ca(2+)ATPase protein expression and an 85% (P<0.01) reduction in the extent of phosphorylation of phospholamban in the left ventricular myocardium of HF patients in atrial fibrillation compared with those in sinus rhythm. CONCLUSIONS: Together, these data demonstrate that ventricular rhythmicity contributes significantly to excitation-contraction coupling by altering the expression and activity of key calcium-handling proteins. These data suggest that control of rhythm may be of benefit in patients with HF.

Heat shock proteins protect against ischemia and inflammation through multiple mechanisms.

After heat shock or other metabolic stress, heat shock proteins (Hsps) are expressed at high levels in all tissues and cells. The highly inducible 70 kDa heat shock protein (Hsp70) is associated with improved post-ischemic myocardial contractile recovery. Similarly, the small 27 kDa heat shock protein (Hsp27), that is abundant in muscle, is also linked with improved myocardial function after ischemic injury. Various Hsps have pro-survival functions that include chaperone, anti-apoptotic and/or anti-inflammatory activity. In this review we will summarize our understanding of myocardial protection and present evidence for protection having time dependent aspects that appear to be stimulus dependent.

Insulin-induced myocardial protection in isolated ischemic rat hearts requires p38 MAPK phosphorylation of Hsp27.

Six hours after insulin treatment, hearts express heat shock protein 70 (Hsp70) and have improved contractile function after ischemia-reperfusion injury. In this study we examined hearts 1 h after insulin treatment for contractile function and for expression of Hsp70 and Hsp27. Adult, male Sprague-Dawley rats were assigned to groups: 1) sham, 2) control, 3) insulin injected (200 microU/g body wt), 4) heat shock treated (core body temperature, 42 degrees C for 15 min), and 5) heat shock and insulin treated. At 1 h after these treatments, hearts were isolated, equilibrated to Langendorff perfusion for 30 min, and then subjected for 30 min no-flow global ischemia (37 degrees C) followed by 2 h of reperfusion. Insulin-treated hearts had significantly increased contractile function compared with control hearts. At 1 h after insulin treatment, a minimal change in Hsp70 and Hsp27 content were detected. By 3 h after insulin treatment, a significant increase in Hsp70, but not Hsp27, was detected by Western blot analysis. By immunofluorescence, minimal Hsp70 was detected in insulin-treated hearts, whereas Hsp27 was detected in all hearts, indicative of its constitutive expression. Phosphospecific isoforms of Hsp27 were detected in insulin-, heat shock-, and heat shock and insulin-treated hearts. After ischemia and reperfusion, the insulin-treated hearts had significantly elevated levels of phosphorylated Hsp27. Inhibition of p38 MAPK with SB-203580 blocked the insulin-induced phosphorylation of Hsp27 and the improved functional recovery. In conclusion, insulin induces an apparent rapid phosphorylation of Hsp27 that is associated with improved functional recovery after ischemia-reperfusion injury.

Insulin induces myocardial protection and Hsp70 localization to plasma membranes in rat hearts.

Insulin induces the expression of the 70-kDa heat shock protein (Hsp70) in rat hearts. In this study, we examined insulin- and heat shock-treated hearts for improved contractile recovery after 30 min of ischemia, activation of the heat shock transcription factor, and localization of the Hsp70 in relation to dystrophin and alpha-tubulin. Adult male Sprague-Dawley rats were assigned to groups: 1) control, 2) sham control, 3) insulin injected (200 microU/g body wt), 4) heat shock treated (core body temperature 42 degrees C for 15 min), and 5) heat shock and insulin treated. Six hours later, hearts were isolated for Langendorff perfusion to determine cardiac function, or myocardial tissues were collected and prepared for either electrophoretic mobility shift assay, Western blot analysis, or immunofluorescence microscopy. Insulin treatment with 6 h of recovery enhances postischemic myocardial recovery of contractile function and increases Hsp70 expression through activation of the heat shock transcription factor. Insulin-treated hearts had elevated levels of Hsp70, particularly in the membrane fraction. In contrast, heat-shocked hearts had elevated levels of Hsp70 in the cytosol, membrane, and pellet fractions. After insulin treatment, Hsp70 was mostly colocalized to the plasma membrane with dystrophin. In contrast, after heat shock, Hsp70 was localized mostly between cardiomyocytes in apparent vascular or perivascular elements. The localization of Hsp70 is dependent on the inducing stimuli of either heat shock or insulin treatment. The cell membrane versus vascular localization of Hsp70 suggests the interesting possibility of functionally distinct roles for Hsp70 in the heart, whether induced by insulin or heat shock treatment.

Insulin potentiates expression of myocardial heat shock protein 70.

OBJECTIVE: Since insulin stimulates nitric oxide (NO) production and an increase in NO following heat shock is required for myocardial heat shock protein 70 (Hsp70) synthesis, we hypothesized that insulin would enhance myocardial Hsp70 synthesis by augmenting NO signaling. We examined whether a physiologic dose of insulin increased myocardial Hsp70 in unstressed and heat shock treated rats. METHODS: Adult male Sprague-Dawley rats were assigned to groups: (1) control, (2) insulin injected (200 microU/gm body weight), (3) heat shock treated (core body temperature 42 degrees C for 15 min), (4) heat shock and insulin treated, (5) L-nitroarginine methyl ester (L-NAME) and heat shock and insulin treated, (6) sodium nitroprusside (SNP) and heat shock and insulin treated. Six hours later, myocardial Hsp70 content and localization was analyzed. RESULTS: Hsp70 was increased in heat shock treated hearts (120.6+/-16.8 ng/mg protein, P < 0.001) vs. control (12.9+/-2.0 ng/mg protein), or insulin treated hearts (15.5+/-0.83 ng/mg protein). In addition, Hsp70 was increased in the heat shock and insulin treated hearts (164.4+/-7.53 ng/mg protein) compared to control, insulin only (P = 0.001) or heat shock only treated hearts (P = 0.01). L-NAME did not abolish the insulin induced increase in Hsp70 in heat shocked hearts (195.2+/-13.4 ng/mg protein, P = 0.21) and SNP did not further enhance Hsp70 in the insulin and heat shocked group (188.9+/-8.2 ng/mg protein, P = 0.71). Western analysis and confocal microscopy revealed a lowlevel expression of myocardial Hsp70 in response to insulin. Hsp70 was localized primarily in blood vessels after insulin or heat shock treatments. CONCLUSIONS: Insulin caused a low-level expression of myocardial Hsp70 and potentiated Hsp70 synthesis in response to heat shock. The ability of insulin to potentiate Hsp70 after heat shock is independent of NO signaling as it was not altered by either LNAME or SNP pretreatment. Blood vessels appear to be the primary site of Hsp70 after insulin or heat shock treatment.