In contrast to binding experiments, which measure occupancy of receptors, the Xenopus oocyte expression system we use can yield direct measures of transduction of that occupancy into physiological response. Thus the pharmacological measure of intrinsic activity is easily accessible by this system. In addition, the fast response of the oocyte permits kinetic measurements that can address questions of desensitization, rate of recovery from blockade, and rates for activation of the receptor-transduction machinery. Wild-type (WT) and two mutant mu receptors, produced previously, were expressed in oocytes along with a potassium channel that serves as a reporter for activation. In voltage clamped oocytes, agonists produce inward currents. We previously showed that wild type and mutants H297N and H297Q transduce ligands differently such that alkaloid partial agonists behave as more nearly full agonists and alkaloid antagonists behave as partial agonists. We continued in the comparison between WT and mutant receptors (particularly H297N) this year using agonist pulse experiments to estimate rates of recoveries from drugs. The dichotomy between WT and H297N seen in concentration response curves was also apparent in the kinetic experiments. Generally the H297N mutant recovered faster from antagonists than did the WT receptor. For example, recovery from diprenorphine was 100 times faster at H297N. Conversely, the H297N mutant recovered more slowly from agonists than did the WT receptor. The question of how this phenomenon relates to the boosting of partial agonism seen last year, how it relates to the hydrophobicities of the drugs, and whether membrane reservoirs participate in these effects are continuing topics of interest.
NIH ExPorter
NIH ExPorter