A 3-year-old boy was admitted to the emergency room after e generalized tonic colonic seizure. His mother reported that the boy apparently ingested several tablets of propranolol (a beta-blocking drug) found in his father drawer. The boy received an IM injection of glucagon, a hormone that can bind to glucagon receptors in the heart, so causing a significant increase in heart contractility. In this way It can counteract the decrease in heart contractility brought about by a toxic dose of a beta-blocker. Which of the following terms best defines the antagonism between glucagon and beta-blockers?