Characterization of catecholamine uptake2 in isolated cardiac myocytes.

Extraneuronal catecholamine uptake was investigated in isolated quiescent rat myocardial cells. By administration of (3H-)(-)-noradrenaline concentration of 22 nmol/l up to 1000 mumol/l the following data were obtained: (1) The KM of the uptake process amounted to 260 mumol/l, the Vmax to 4.24 nmol/ (10 min x mg Protein) corresponding to 179 nmol/(min x gWWt)(WWT = Wet Weight). (2) The uptake was largely inhibited by the uptake2-inhibitors corticosterone (100 mumol/l), isoprenaline (IC50 = 30.6 mumol/l), and O-methylisoprenaline (IC50 = 2.1 mumol/l), but not by the uptake1-inhibitors cocaine (100 mumol/l) and desipramine (10 mumol/l). (3) The 'affinity'-values KM and IC50 closely agreed with those already known, but the Vmax-value was higher than those obtained in whole rat hearts by a factor of at least 1.79. This is caused presumably by the voltage dependence of the uptake mechanism and the resulting inhibition of uptake2 during the periods of depolarisation in beating hearts of other studies.