A 52-year-old male with a history of exertional angina pectoris has experienced episodes occurring at 5 AM daily for the past 2 weeks, with prolonged pain duration. During hospitalization, the episodes were associated with a heart rate of 50 bpm, 4-5 premature beats/min, blood pressure of 95/60 mmHg (12.5/8 kPa), and ST-segment elevation in leads II, III, and aVF on ECG. Nifedipine was added, and no further episodes occurred. The mechanism of nifedipine's action in this case is: