A 52-year-old man with hypertension is started on atenolol 50 mg daily. After 2 weeks, his blood pressure remains 160/95 mmHg despite good adherence. His heart rate is 58 bpm. The physician increases the dose to 100 mg daily. After another 2 weeks, blood pressure is still 160/92 mmHg with heart rate now 52 bpm. Which pharmacodynamic principle best explains the lack of additional blood pressure reduction despite dose escalation and further heart rate reduction?