http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Positive inotropic effects of morphine in isometric rat papillary muscle preparations following isolation in high magnesium cardioplegia
xiii, 104 leaves : illustrations ; 29 cm
Possible explanations for the positive inotropic effects of morphine on isometric rat papillary muscle preparations isolated in high magnesium cardioplegia were studied. Muscles isolated in high magnesium (32 mEq/L) cardioplegia then challenged with morphine (0.1 mg/ml) responded with an increase in peak developed tension (29% increase from baseline), unlike preparations isolated in balanced salt solution, where morphine (0.1 mg/ml) and magnesium (1.0 mEq/L) individually both caused a decrease in peak developed tension (13% and 16% decrease from baseline respectively). However the same concentrations of morphine and magnesium combined in balanced salt solution isolated preparations resulted in increased peak developed tension (22% increase from baseline). These phenomena were not unique to one set of concentrations tested. Proportionate decreases (by 11 percent) of morphine and magnesium resulted in increased peak developed tension (13% increase from baseline). Similarly, an 11% decrease of the magnesium in the isolation cardioplegia and an 11% decrease of the added morphine resulted in an average increase of 20% in peak developed tension from baseline. The use of the calcium channel blocker, verapamil (0.125 micrograms/ml), after isolation in the cardioplegia and prior to the addition of morphine (0.1 mg/ml) effectively blocked the positive inotropic response. Also, the T1/2 values (half-time of relaxation from peak tension) calculated from recorded tension responses in each of the experiments did not change from baseline, a pattern of response consistent with an agent which causes increased extracellular calcium concentration. The result is an increase in peak developed tension. It was concluded that morphine and magnesium preferentially complex with each other when combined. The morphine/magnesium complexing decreases the amount of magnesium available to antagonize calcium. The amount of calcium and morphine complexing is also decreased when morphine and magnesium complex in these preparations. The net effect is, apparently, an increase in extracellular calcium concentration, resulting in increased peak developed tension.