What isotope pair used to determine the age of fossils

Oxytocin and cholecystokinin are transported through the bloodstream and arrive at the uterus at the same time. oxytocin causes the uterus to contract, whereas cholecystokinin has no effect. why? oxytocin blocks the activity of cholecystokinin. oxytocin is at a higher concentration in the bloodstream than cholecystokinin. oxytocin can diffuse through the plasma membrane of uterine cells, whereas cholecystokinin cannot. oxytocin alters gene expression patterns, whereas cholecystokinin does not. the uterus has receptors for oxytocin, but not for cholecystokinin.