Adrenergic stimulation of Rb+ uptake in pig hearts in vivo assessed by 87Rb MRS

It was previously shown that the adrenergic agonist dobutamine stimulates Rb(+) uptake in isolated pig hearts. In the present work we examined whether dobutamine can increase Rb(+) uptake rate in vivo. Open-chest domestic pigs (N = 14) were used under general anesthesia. The surface coil was placed against the anterior left ventricular wall to obtain (87)Rb spectra using a spectrometer interfaced with the 7T, 30-cm horizontal bore magnet. RbCl was infused at the rate of 1.35 +/- 0.14 mmol/kg/hr without or with dobutamine (0.6 mg/kg/hr) over a 60-min period, and then the infusions were terminated. The rate constant (k x 10(3), from 13 +/- 2.4 to 36 +/- 12 min(-1)) and Rb(+) influx rate (from 2.5% to 4.8%/min) were increased by dobutamine despite lower plasma [Rb(+)] (0.59 +/- 0.16 vs. 0.84 +/- 0.24 mM in control), while the tissue/plasma Rb ratios were identical (38 +/- 9). Heart rate (HR) and systolic blood pressure (SBP) were increased from 106 +/- 9 to 161 +/- 15 bpm (+52%) and from 78 +/- 7 to 93 +/- 11 mmHg (+19%), respectively. The stimulation of Rb(+) uptake by dobutamine is consistent with the activation of Na(+)/K(+) ATPase previously observed in isolated hearts. However, the 50% increase in HR and the double coronary flow caused by the SBP increase and vasodilatation may also contribute to this effect.