Improvement of cardiac performance by carboxylic ionophore monensin in greyhound and mongrel dogs

Elevated pulmonary artery pressure and brain natriuretic peptide in high altitude pulmonary edema susceptible non-mountaineers

Exaggerated pulmonary pressor response to hypoxia is a pathgonomic feature observed in high altitude pulmonary edema (HAPE) susceptible mountaineers. It was investigated whether measurement of basal pulmonary artery pressure (Ppa) and brain natriuretic peptide (BNP) could improve identification of HAPE susceptible subjects in a non-mountaineer population. We studied BNP levels, baseline hemodynamics and the response to hypoxia (FIo2 = 0.12 for 30 min duration at sea level) in 11 HAPE resistant (no past history of HAPE, Control) and 11 HAPE susceptible (past history of HAPE, HAPE-S) subjects. Baseline Ppa (19.31 ± 3.63 vs 15.68 ± 2.79 mm Hg, p < 0.05) and plasma BNP levels (52.39 ± 32.9 vs 15.05 ± 9.6 pg/ml, p < 0.05) were high and stroke volume was less (p < 0.05) in HAPE-S subjects compared to control. Acute hypoxia produced an exaggerated increase in heart rate (p < 0.05), mean arterial pressure (p < 0.05) and Ppa (28.2 ± 5.8 vs 19.33 ± 3.74 mm Hg, p < 0.05) and fall in peripheral oxygen saturation (p < 0.05) in HAPE-S compared to control. Receiver operating characteristic (ROC) curves showed that Ppa response to acute hypoxia was the best variable to identify HAPE susceptibility (AUC 0.92) but BNP levels provided comparable information (AUC 0.85). BNP levels are easy to determine and may represent an important marker for the determination of HAPE susceptibility.

Effect of ionophore monensin on anaesthetized dogs in hemorrhagic hypotension

AIM

Progressive decompensation and irreversible cellular injury from the severe hemorrhagic shock can be reversed therapeutically with aggressive and timely resuscitation by administering volume expanding fluids. The possibility of the use of rapidly acting pressor agents which can effectively maintain the blood pressure and peripheral perfusion (without volume resuscitation) and prevent the progression of shock and providing in regular time interval for volume replacement has not been looked into. In the present investigation, two pressor agents, dopamine and a long acting carboxylic ionophore monensin, were examined for their potential role in hemorrhagic hypotension produced experimentally in dogs.

METHODS

Two groups of animals were studied: (a) In the first group (n=10) 20% of the total blood volume i.e. moderate hemorrhage was induced. (b) In the second group (n=10) 35% of the total blood volume i.e. severe hemorrhage was induced. After 30 min of induction of hemorrhage both the groups were given intravenous infusion of dopamine (20 microgm/kg/min) for 10 min. Then after 30 min of stopping dopamine infusion, both the groups were given intravenously a single bolus dose (50 microgm/kg) of monensin.

RESULTS

In both moderate and severe hemorrhage monensin injection produced significant increases in systolic arterial blood pressure, mean arterial blood pressure, cardiac output and stroke volume. The effects persisted for 1 h of administration. In severe hemorrhage dopamine infusion produced significant decrease in mean arterial blood pressure, diastolic blood pressure and pH. Dopamine produced a significant rise in heart rate also while monensin did not have such an effect.

CONCLUSION

Monensin produced a rapid and sustained recovery of the arterial blood pressure without requiring adequate preload. Dopamine failed to produce such an effect because it requires adequate circulating blood volume for its pressor action. Thus monensin has the potential to prevent progression of shock and improving the prognosis in management of hemorrhagic hypotension.

Pharmacological actions of monovalent ionophores on spontaneously beating rabbit sino-atrial nodal cells

The effects of sodium (monensin) and potassium (nigericin and lonomycin A) ionophores on the spontaneous activity in rabbit sino-atrial (SA) nodal cells were investigated using microelectrode and whole-cell voltage-clamp techniques. In the multicellular preparations, the ionophores produced a negative chronotropic effect in a concentration-dependent manner, and at 3x10(-5) M significantly decreased the amplitude and duration of action potentials and enhanced the maximum rate of depolarization. The ionophores elicited dysrhythmias and then a sinus arrest often occurred. These responses were reversible. In whole-cell clamp experiments, monensin enhanced the L-type Ca2+ current (ICa), whereas lonomycin A and nigericin inhibited ICa. The fast component of the inactivation phase for ICa was decreased by the ionophores, but the slow component was unaffected. The activation and inactivation kinetics (d infinity and f infinity) were not altered. The ionophores did not affect the hyperpolarization-activated inward current. Monensin inhibited the delayed rectifier K+ current (I(K)), but lonomycin A and nigericin increased I(K). Its activation kinetics shifted in the depolarizing direction. The effects on the ionic currents were irreversible. Monensin (30 microM) increased cellular Ca2+ concentration ([Ca2+]i), using Ca2+ -sensitive fluorescent dye (fura-2). These results indicate that the monovalent ionophores depress the action potentials and produce a negative chronotropic effect due to direct and indirect modulations of the ionic currents and the [Ca2+]i level in rabbit SA nodal cells.