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Cardiovascular Research 1987 21(10):772-778; doi:10.1093/cvr/21.10.772
© 1987 by European Society of Cardiology
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Copyright © 1987, European Society of Cardiology

Effects of verapamil, dibutyryl cyclic AMP, and extracellular calcium on intracellular free calcium concentrations in myocytes isolated from rat ventricles

AMANDA M AULD, JAMES ASPINALL and GREGORY J BARRITT

From the Department of Biochemistry and Chemical Pathology, Flinders University School of Medicine, Flinders Medical Centre, Bedford Park, South Australia

The concentration of free calcium in the myoplasm of myocytes isolated from rat ventricles was measured using quin2. Incubation of myocytes with the acetoxymethylester of quin2 (50 µmol·litre–1) for 45 min was required to give effective loading with quin2. At 1 mmol·litre–1extracellular calcium free calcium was 210(17) (n = 5) nmol·litre–1. Depolarisation of myocytes by addition of 40 mmol·litre–1 potassium caused a threefold increase in free calcium. Increases in extracellular calcium from 0.25 to 2.0 mmol·litre–1 caused a twofold increase in free calcium in polarised (resting) myocytes and a 2.5-fold increase in cells depolarised with potassium. The ability of verapamil to inhibit the electrically stimulated contraction of myocytes was dependent on the stimulation voltage. Half-maximal inhibition was given by 0.7 and 2 µmol·litre–1 verapamil at 50 and 100 V stimulation respectively. High concentrations of verapamil completely inhibited potassium induced increases in the fluorescence of quin2 loaded cells. Half-maximal inhibition was observed at 0.5 µmol·litre–1 verapamil. The calcium agonist CGP 28392 enhanced potassium induced fluorescence of quin2 loaded cells. Dibutyryl cyclic adenosine monophosphate and adrenaline increased the proportion of rod shaped cells that contracted in response to stimulation by low, but not high, voltage. Dibutyryl cyclic AMP caused a threefold increase in the potassium induced increase in free calcium. It is concluded that verapamil decreases and that calcium agonists, dibutyryl cyclic AMP, and increases in extracellular calcium increase free calcium, as monitored by intracellular quin2.

KEYWORDS calcium physiology; heart physiology; isolated cardiac myocytes; myoplasmic free calcium concentration; verapamil; dibutyryl cyclic AMP

Address for correspondence and reprints: Dr G J Barritt, Department of Biochemistry and Chemical Pathology, Flinders Medical Centre, Bedford Park, South Australia 5042. Australia.


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