AbstractWe investigated the effect of albumin (6) on the (±)‐isoprenaline‐induced relaxation of strips of isolated rat uterus and its antagonism by (−)‐propranoioi. The mean isoprenaline EC50in the presence of albumin was significantly less than that in the absence of albumin (geometric mean 1.94 ± 3.33 versus 3.21 ± 3.50 nM, respectively; n = 14;p= 0.006, pairedttest). This indicates enhancement of isoprenaline activity by albumin which could not be explained by protein binding, as this would have reduced activity. Geometric mean controlKBvalues for inhibition of isoprenaline by propranolol at 26.8 and 500 nM were 0.835 ± 1.68(n = 10) and 0.889 ± 1.60 nM (n = 34), respectively. In the presence of 6 albumin,KBfor propranolol was increased significantly to 13.3 ± 1.8 nM (n = 27,p0.05). Therefore, propranolol activity was as predicted by the unbound drug concentration in Contrast to isoprenaline activity. We conclude that albumin can alterin vitrodrug activity by mechanisms in addition to the reduction of unbound drug
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