Cytoplasmic calcium regulation and the electrocardiogram in patients with primary hyperparathyroidism.

Primary hyperparathyroidism (HPT) is a disease caused by an abnormal cytosolic regulation of calcium concentration [Ca++](i) leading to an increased secretion of parathyroid hormone and thereby increased levels of extracellular calcium. It is well known that the QT-interval measured at electrocardiography (ECG) is shortened in HPT subjects. Whether this is due to an abnormal intracellular handling of calcium also in the heart or to the raised extracellular calcium levels is not known. In order to study the extent to which the deranged extra- and intracellular levels of calcium in HPT patients were related to ECG characteristics, [Ca++](i) was determined in vitro by microfluorometry in surgery-removed parathyroid cells at extracellular calcium concentrations of 0.5 mM and 30.0 mM and ECG was recorded preoperatively in 42 HPT patients and in 15 subjects operated on for atoxic goitre. Serum calcium and plasma-ionized calcium also were measured preoperatively. The QT-interval and ST-segment duration were both shortened in the HPT patients compared to controls (P<0.001). [Ca++](i) at 3.0 mM extracellular calcium divided by that at 0.5 mM was correlated to the QT-interval, when measured at the onset of the T-wave (QoT, r=0.39, P<0.03) and early diastolic phase (end of T-wave to onset of p-wave, r=-0.34, P=0.05). Plasma ionized calcium was related inversely to the QT-interval, when measured to the apex of the T-wave (QaT, r=-0.48, P<0.04), while the non-ionized form of calcium in serum was correlated to the length of the PQ-interval (r=0.53, P<0.02). The study described here showed the altered cytosolic calcium levels in the HPT subjects to be related to QoT-interval and early diastolic phase implying that a defect intracellular handling of calcium could be present also in the heart to cause these ECG changes. On the other hand, the levels of plasma ionized and serum non-ionized calcium were related to ECG characteristics suggesting that the extracellular levels of this ion are important.