Catecholamine transport in isolated lung parenchyma of pig

1 Lung parenchyma strips of the pig incubated at 37 degrees C with [(3)H]-(-)-noradrenaline ([(3)H]-NA) or [(3)H]-(+/-)-isoprenaline ([(3)H]-Iso), accumulated radioactivity via saturable, high affinity uptake processes. Apparent saturation constants (K(m)) for [(3)H]-NA and [(3)H]-Iso were 1.34 x 10(-6) M and 1.63 x 10(-6) M respectively, while apparent transport maxima (V(max)) were 4.86 and 1.63 x 10(-9) mol min(-1) g(-1) respectively.2 Cellular accumulation of radioactivity from radiolabelled catecholamines was greatly reduced by lowering the temperature to 7 degrees C, pretreatment with ouabain (100 muM), phentolamine (15 muM) or phenoxybenzamine (80 muM). However, accumulation of radioactivity derived from ((3)H]-NA was inhibited selectively by cocaine (10 muM) and desipramine (1 muM), while normetanephrine (80 muM) and 3-O-methylisoprenaline (50 muM) caused much greater reductions in cellular radioactivity from [(3)H]-Iso than from ((3)H]-NA. Taken together with information from kinetic studies, the results indicate that these amines are transported by separate uptake processes.3 Cocaine (50 muM) which selectively reduced [(3)H]-NA transport, had no significant effect on the sensitivity (EC(50)) of isolated parenchyma lung strips of the pig to the contractile effects of cumulative concentrations of NA. The catechol-O-methyl transferase (COMT) inhibitor, U-0521 (60 muM), also failed to alter the potency of NA, while normetanephrine (80 muM) caused a 2 fold decrease in potency.4 Phentolamine (15 muM), which reduced the cellular accumulation of radioactivity derived from [(3)H]-Iso by 64%, caused a small potentiation of Iso-induced relaxations of porcine lung strips. Normetanephrine (80 muM) and 3-O-methylisoprenaline (50 muM), which also depressed the accumulation of cellular radioactivity from [(3)H]-Iso by > 50%, caused rightward shifts in Iso concentration-effect curves as a result of beta-adrenoceptor blockade. In sharp contrast, cortisol (80 muM) and U-0521 (60 muM), which caused smaller reductions in the cellular accumulation of radioactivity derived from [(3)H]-Iso, both caused an approximately 9 fold potentiation of responses to Iso in isolated lung strips.5 The results indicate that the major sites of uptake and metabolism of NA in porcine parenchyma strip are remote from alpha-adrenoceptors mediating NA-induced contraction. Similarly, some major sites of uptake of Iso are remote from beta-adrenoceptors mediating Iso-induced relaxation. However, beta-adrenoceptors are apparently in close proximity to a compartment containing COMT activity.