Effects of magnesium cardioplegia on regulation of the porcine coronary circulation

Relevance and Recommendations for the Application of Cardioplegic Solutions in Cardiopulmonary Bypass Surgery in Pigs

Cardioplegic solutions play a major role in cardiac surgery due to the fact that they create a silent operating field and protect the myocardium against ischemia and reperfusion injury. For studies on cardioplegic solutions, it is important to compare their effects and to have a valid platform for preclinical testing of new cardioplegic solutions and their additives. Due to the strong anatomical and physiological cardiovascular similarities between pigs and humans, porcine models are suitable for investigating the effects of cardioplegic solutions. This review provides an overview of the results of the application of cardioplegic solutions in adult or pediatric pig models over the past 25 years. The advantages, disadvantages, limitations, and refinement strategies of these models are discussed.

Role of Coronary Myogenic Response in Pressure-Flow Autoregulation in Swine: A Meta-Analysis With Coronary Flow Modeling

Myogenic responses (pressure-dependent contractions) of coronary arterioles play a role in autoregulation (relatively constant flow vs. pressure). Publications on myogenic reactivity in swine coronaries vary in caliber, analysis, and degree of responsiveness. Further, data on myogenic responses and autoregulation in swine have not been completely compiled, compared, and modeled. Thus, it has been difficult to understand these physiological phenomena. Our purpose was to: (a) analyze myogenic data with standard criteria; (b) assign results to diameter categories defined by morphometry; and (c) use our novel multiscale flow model to determine the extent to which ex vivo myogenic reactivity can explain autoregulation in vivo. When myogenic responses from the literature are an input for our model, the predicted coronary autoregulation approaches in vivo observations. More complete and appropriate data are now available to investigate the regulation of coronary blood flow in swine, a highly relevant model for human physiology and disease.

Effect of cardioplegic and organ preservation solutions and their components on coronary endothelium-derived relaxing factors

Cardioplegic (and organ preservation) solutions were initially designed to protect the myocardium (cardiac myocytes) during cardiac operation (and heart transplantation). Because of differences between cardiac myocytes and vascular (endothelial and smooth muscle) cells in structure and function, the solutions may have an adverse effect on coronary vascular cells. However, such effect is often complicated by many other factors such as ischemia-reperfusion injury, temperature, and perfusion pressure or duration. To evaluate the effect of a solution on the coronary endothelial function, a number of points should be taken into consideration. First, the overall effect on endothelium should be identified. Second, the effect of the solution on the individual endothelium-derived relaxing factors (nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor) must be distinguished. Third, the effect of each major component of the solution should be investigated. Lastly, the effect of a variety of new additives in the solution may be studied. Based on available literature these issues are reviewed to provide information for further development of cardioplegic or organ preservation solutions.

The Effect of Cardiopulmonary Bypass on the Expression of Inducible Nitric Oxide Synthase, Endothelial Nitric Oxide Synthase, and Vascular Endothelial Growth Factor in the Internal Mammary Artery

OBJECTIVE

Endothelial function of a vessel may be impaired by local or systemic inflammation initiated by cardiopulmonary bypass (CPB) during coronary artery bypass graft (CABG) surgery. The present study was designed to investigate the early effects of CPB on nitric oxide production and vascular endothelial growth factor (VEGF) expression in internal mammary artery (IMA).

DESIGN

Prospective study.

SETTING

University hospital.

PARTICIPANTS

Twenty patients who were scheduled for elective CABG with CPB.

INTERVENTIONS

IMA sections were studied immunohistochemically from these patients. The samples were taken from the distal end of the IMA before the institution of CPB and just before the construction of the IMA-left anterior descending artery anastomosis.

MEASUREMENT AND MAIN RESULTS

After CPB, VEGF and endothelial nitric oxide synthase immunoreactivity increased significantly when compared with baseline values in the endothelium (p = 0.0156, p = 0.0313) and adventitia (p = 0.0313, p = 0.0001), respectively. No significant change was observed in inducible nitric oxide synthase immunoreactivity.

CONCLUSIONS

The increase in eNOS expression may have been induced by the inflammation caused by CPB.

Magnesium-flush infusion into the aortic root just before reperfusion reduces the requirement for internal defibrillation and early post-perfusion arrhythmias

Pre- and post-operative administration of magnesium has beneficial effects on post-operative ischaemia and reperfusion arrhythmias, but few studies have examined whether intra-operatively administered magnesium can prevent the effects of intra-operative arrhythmias. The aim of this randomized, double-blind study was to compare the effects of intra-operative magnesium or placebo on intra-operative arrhythmias in patients undergoing coronary bypass grafting. Patients received a flush infusion of magnesium or placebo into the aortic root before cross-clamp removal. The results showed that rate of spontaneous resumption of a cardiac rhythm was significantly higher, and number of shocks for defibrillation, energy requirement for defibrillation and rate of intra-operative ventricular tachyarrhythmias were significantly lower in the magnesium group, compared with the placebo group. The differences in need for temporary pacing, and in serum magnesium levels, were not significant. Intra-operative administration of magnesium has beneficial effects on the outcome of surgery. Larger, multicentre clinical investigations should now be undertaken.

Protective effect of magnesium on the endothelial function mediated by endothelium-derived hyperpolarizing factor in coronary arteries during cardioplegic arrest in a porcine model

OBJECTIVES

Hyperkalemia in cardioplegia impairs the function mediated by endothelium-derived hyperpolarizing factor. This study examined the effect and mechanism of magnesium ion on the relaxation mediated by endothelium-derived hyperpolarizing factor.

METHODS

In the isometric force study, porcine coronary microarteries in a myograph (diameter 200-450 microm) were incubated in Krebs solution (subgroups Ia, IIa, and IIIa), potassium ion (20 mmol/L, subgroups Ib, IIb, and IIIb), magnesium ion (16 mmol/L, subgroups Ic, IIc, and IIIc), or potassium ion plus magnesium ion (subgroups Id, IId, and IIId) for 1 hour at 37 degrees C in group I or II, followed by washout for 45 minutes in group III (n = 8). Relaxation to bradykinin (groups I and III) or sodium nitroprusside (group II) in U(46619)-stimulated contraction was established. In the electrophysiologic study, the membrane potentials of single smooth muscle cells of arteries were measured by microelectrode after superfusion with the previously described solutions (subgroups IVa-IVc).

RESULTS

In group I, 20-mmol/L potassium ion greatly reduced the bradykinin-induced relaxation (35.0% +/- 4.9% vs 86.0% +/- 5.3%, P <.001), which was significantly restored by magnesium ion (51.9% +/- 4.0%, P =.017). In groups II and III, the bradykinin- or nitroprusside-induced relaxation had no significant differences. In group IV, potassium ion depolarized the smooth muscle and decreased the bradykinin-induced hyperpolarization (-72.0 +/- 1.5 vs -61.7 +/- 0.7 mV, n = 7, P <.001), which was significantly restored by magnesium ion (-68.0 +/- 2.5 mV vs -72.5 +/- 1.5 mV, n = 6, P =.029).

CONCLUSIONS

Magnesium ion, either alone or added to hyperkalemic solutions, preserves or helps to restore the endothelial function mediated by endothelium-derived hyperpolarizing factor. The mechanism is related to preservation of the membrane hyperpolarization and reversal of the potassium-induced membrane depolarization of the smooth muscle cell.

Regulation of coronary myoplasmic Ca(2+)-myosin light chain phosphorylation pathway and vasomotor tone: hyperpolarizing versus depolarizing cardioplegia

BACKGROUND

This study was designed to compare the effects of hyperpolarizing versus depolarizing cardioplegic solutions on the coronary vasomotor regulation, specifically focusing on coronary myoplasmic Ca2+-myosin light chain (MLC) phosphorylation pathway and beta-adrenergic signal transduction.

METHODS

With the use of an in vitro cardioplegic model, rat coronary microvessels loaded with fura-2 were subjected to simulated cold (20 degrees C) cardioplegia and reperfused with Krebs solution for 60 minutes at 37 degrees C. Cardioplegia consisted of either (1) Krebs solution alone (control), (2) Krebs plus adenosine triphosphate-sensitive potassium channel opener (100 micromol/L pinacidil [PCO-CP]), (3) hyperkalemic cardioplegia (K(+) = 25 mmol/L [K-CP]), or (4) K-CP plus magnesium (Mg(2+) = 25 mmol/L; [K/Mg-CP]).

RESULTS

At the endpoint of the cardioplegic period, K-CP resulted in a significant increase both in [Ca(2+)](i) and in MLC phosphorylation compared with control (both P <.05). In contrast, PCO-CP did not make any significant difference in these indices compared with control. After reperfusion, the relaxation responses to isoproterenol and forskolin after K-CP were significantly reduced (both P <.05 vs control) but were preserved after PCO-CP. K/Mg-CP provided comparable effects to PCO-CP.

CONCLUSIONS

These results suggest that neither an activation of the coronary myoplasmic Ca(2+)-MLC phosphorylation pathway nor beta-adrenergic desensitization seen after exposure to depolarizing cardioplegia occurs with exposure to hyperpolarizing cardioplegia and magnesium-supplemented depolarizing hyperkalemic cardioplegia.

Vascular changes after cardiopulmonary bypass and ischemic cardiac arrest: roles of nitric oxide synthase and cyclooxygenase

Cardiac surgery involving ischemic arrest and extracorporeal circulation is often associated with alterations in vascular reactivity and permeability due to changes in the expression and activity of isoforms of nitric oxide synthase and cyclooxygenase. These inflammatory changes may manifest as systemic hypotension, coronary spasm or contraction, myocardial failure, and dysfunction of the lungs, gut, brain and other organs. In addition, endothelial dysfunction may increase the occurrence of late cardiac events such as graft thrombosis and myocardial infarction. These vascular changes may lead to increased mortality and morbidity and markedly lengthen the time of hospitalization and cost of cardiac surgery. Developing a better understanding of the vascular changes operating through nitric oxide synthase and cyclooxygenase may improve the care and help decrease the cost of cardiovascular operations.

Intracellular free calcium accumulation in ferret vascular smooth muscle during crystalloid and blood cardioplegic infusions

OBJECTIVE

The effects of magnesium- and potassium-based crystalloid and blood-containing cardioplegic solutions on coronary smooth muscle intracellular free calcium ([Ca2+]i) accumulation and microvascular contractile function were examined.

METHODS

Isolated ferret hearts were subjected to hyperkalemic (25 mmol/L K+) blood cardioplegic infusion, hypermagnesemic (25 mmol/L Mg2+, K+-free) crystalloid cardioplegic infusion, or hyperkalemic crystalloid cardioplegic infusion for 1 hour. Coronary arterioles were isolated, cannulated, and loaded with fura 2. Reactivity and [Ca2+]i were assessed with videomicroscopy. [Ca2+]i was measured at baseline and after application of 50 mmol/L KCl. In addition, [Ca2+]i and vascular contraction were measured during exposure to Mg2+ and K+ cardioplegic solution at both 4 degrees C and 37 degrees C.

RESULTS

From a baseline [Ca2+]i of 177 +/- 52 nmol/L, K+ cardioplegic infusion (302 +/- 80 nmol/L potassium) markedly increased [Ca2+]i, whereas blood cardioplegic infusion (214 +/- 53 nmol/L) and Mg2+ cardioplegic infusion (180 +/- 42 nmol/L) did not alter [Ca2+]i. Although a difference between groups in percentage contraction after application of 50 mmol/L KCl was not observed, [Ca2+]i increased significantly more in vessels in the control group (764 +/- 327 nmol/L) and the K+ crystalloid cardioplegic infusion group (698 +/- 215 nmol/L) than in vessels in the blood cardioplegic infusion group (402 +/- 45 nmol/L) and the Mg2+ cardioplegic infusion group (389 +/- 80 nmol/L). Mg2+ cardioplegic solution induced no microvascular contraction at either 4 degrees C or 37 degrees C, nor was an increase in [Ca2+]i observed. K+ cardioplegic solution induced microvascular contraction at 37 degrees C but not at 4 degrees C; it increased [Ca2+]i at both 4 degrees C and 37 degrees C.

CONCLUSION

An Mg2+-based cardioplegic solution, or appropriate Mg2+ or blood supplementation of a K+ crystalloid cardioplegic solution, may decrease the accumulation of [Ca2+]i in the vascular smooth muscle during ischemic arrest.

Coronary microvascular protection with mg2+: effects on intracellular calcium regulation and vascular function

The use of Mg2+-supplemented hyperkalemic cardioplegia preserves microvascular function. However, the mechanism of this beneficial action remains to be elucidated. We investigated the effects of Mg2+ supplementation on the regulation of intracellular calcium concentration ([Ca2+]i) and vascular function using an in vitro microvascular model. Ferret coronary arterioles (80-150 micrometer in diameter) were studied in a pressurized (40 mmHg) no-flow, normothermic (37 degrees C) state. Simultaneous monitoring of internal luminal diameter and [Ca2+]i using fura 2 were made with microscopic image analysis. The microvessels (n = 6 each group) were divided into four groups according to the content of MgCl2 (nominally 0, 1.2, 5.0, and 25.0 mM) in a hyperkalemic cardioplegic solution ([K+] 25.0 mM). After baseline measurements, vessels were subjected to 60 min of hypoxia with hyperkalemic cardioplegia (equilibrated with 95% N2-5% CO2) containing each concentration of Mg2+ ([Mg2+]) and were then reoxygenated. During hyperkalemic cardioplegia, [Ca2+]i increased in a time-dependent manner in all groups. In the lower [Mg2+] cardioplegia groups, [Ca2+]i was significantly increased at the end of the 60-min cardioplegic period (247 +/- 44 nM and 236 +/- 49 nM in [Mg2+] 0 and 1.2 mM groups, respectively; both P < 0.05 vs. baseline) with 19.6-17.2% vascular contraction. Conversely, there was no significant [Ca2+]i increase in the higher [Mg2+] cardioplegia groups and less vascular contraction (5.4-4.1%, both P < 0.05 vs. [Mg2+] 1.2 mM group). After reperfusion, agonist (U-46619, thromboxane A2 analog)-induced vascular contraction was significantly enhanced in the lower [Mg2+] cardioplegia groups (both P < 0.05 vs. control) but was normalized in the higher [Mg2+] cardioplegia groups. Intrinsic myogenic contraction was significantly decreased in the lower [Mg2+] cardioplegia groups (both P < 0.05 vs. control) but was preserved in the higher [Mg2+] cardioplegia groups. These results suggest that supplementation of the solution with >5.0 mM [Mg2+] may prevent hyperkalemic cardioplegia-related intracellular Ca2+ overloading and preserve vascular contractile function in coronary microvessels.

Comparative effects of continuous warm blood and intermittent cold blood cardioplegia on coronary reactivity

BACKGROUND

Cardioplegia is known to affect coronary vascular reactivity. We examined the effects of intermittent cold and continuous warm blood cardioplegia on beta-adrenoceptor-mediated, adenosine triphosphate-sensitive K+ (K+ATP)-channel-mediated, and endothelium-dependent relaxation and on the myogenic tone of coronary arterioles.

METHODS

Pigs were placed on cardiopulmonary bypass. Hearts were arrested for 1 hour with a cold blood cardioplegic solution administered intermittently (n = 12; iCB-CP) or with a warm blood cardioplegic solution delivered continuously (n = 12; cWB-CP). Selected hearts (n = 6 in each group) were then reperfused for 1 hour. In vitro relaxation responses of precontracted microvessels (50 to 160 microns) were studied in a pressurized no-flow state.

RESULTS

Relaxation in response to isoproterenol (beta-adrenergic agonist) was similar after iCB-CP and cWB-CP, whereas forskolin (adenylate cyclase activator)-induced relaxation was impaired more after iCB-CP than after cWB-CP. After reperfusion the respective responses were similar. Both iCB-CP and cWB-CP preserved receptor-mediated, endothelium-dependent relaxation in response to adenosine, 5'-diphosphate; non-receptor-mediated endothelium-dependent relaxation in response to A23187; endothelium-independent cyclic guanosine monophosphate-mediated relaxation in response to sodium nitroprusside, and K+ATP-channel-mediated relaxation. Relaxations in response to 8-bromo-cyclic guanosine monophosphate (a cyclic guanosine monophosphate-dependent protein kinase activator) and to 8-bromo-cyclic adenosine monophosphate (a cyclic adenosine monophosphate-dependent protein kinase activator) were impaired after iCB-CP alone and after reperfusion, whereas the respective responses were not affected after cWB-CP. Myogenic tone was decreased similarly after iCB-CP and cWB-CP but was not further altered after reperfusion. Cardiac function was similar after iCB-CP and cWB-CP.

CONCLUSIONS

These results suggest that cWB-CP is similar to iCB-CP in its ability to preserve endothelium-dependent relaxation and K+ATP-channel function. The superior preservation of beta-adrenergic-cyclic adenosine monophosphate-mediated coronary responses after cWB-CP is brief and associated with minimal improvement of myocardial function and myogenic tone.