Acute effect of parathyroid hormone-(1-34) fragment on blood pressure in rats fed a low calcium diet

Osteosclerotic prostate cancer metastasis to murine bone are enhanced with increased bone formation

Spontaneous development of osteoblastic lesions of prostate cancer (PCa) in mice is modeled by orthotopic (intraprostatic) deposition of neoplastic cells followed by an extremely long latency associated with low incidence of spontaneous bone metastasis. Intracardial injection results in overt bone metastases only with osteoclastic PCa cells (i.e., PC-3). Herein, we report that androgen independent osteoblastic PCa cells readily colonize bone when in a high remodeling state. SCID/Beige mice were subjected to periods of intermittent human parathyroid hormone 1-34 (hPTH) exposure, followed by an intracardiac infusion of osteoblastic C4-2 PCa cells. At the time of PCa infusion, analysis of bone turnover markers from mice treated with hPTH revealed significant increases in osteocalcin (55.06 +/- 7.5 vs. 74.01 +/- 18.5 ng/ml) and TRAcP-5b (3.3 +/- 0.6 vs. 4.81 +/- 0.8 U/l), but no change in type I collagen C-terminal teleopeptide levels relative to control mice. Analysis of femoral cancellous bone architecture revealed significant increases in bone mineral density, trabecular thickness (0.056 +/- 0.002 vs. 0.062 +/- 0.001 mm) and porosity, but significant decreases in connectivity density and trabecular number in hPTH treated mice relative to controls. By 8 weeks post-infusion, 70% of mice pre-treated with hPTH demonstrated detectable serum prostate specific antigen (PSAs) ranging between 2 and 18.8 ng/ml. Immuno-histochemical labeling of femurs for PSA and pan-Cytokeratin revealed the presence of significant tumor cell nests in marrow and trabecular spaces. These results suggest that: (1) local bone physiology is an important factor for developing osteoblastic/sclerotic PCa bone metastases in murine hosts; (2) the establishment of osteosclerotic PCa bone metastases in mice is enhanced by alterations that drive bone formation.

Effect of changes in ionized calcium concentration in arterial blood and metabolic acidosis on the arterial partial pressure of oxygen in dogs

OBJECTIVE

To evaluate the effects of metabolic acidosis and changes in ionized calcium (Ca2+) concentration on PaO2 in dogs.

ANIMALS

33 anesthetized dogs receiving assisted ventilation.

PROCEDURE

Normal acid-base status was maintained in 8 dogs (group I), and metabolic acidosis was induced in 25 dogs. For 60 minutes, normocalcemia was maintained in group I and 10 other dogs (group II), and 10 dogs were allowed to become hypercalcemic (group III); hypocalcemia was then induced in groups I and II. Groups II and IV (5 dogs) were treated identically except that, at 90 minutes, the latter underwent parathyroidectomy. At intervals, variables including PaO2, Ca2+ concentration, arterial blood pH (pHa), and systolic blood pressure were assessed.

RESULTS

In group II, PaO2 increased from baseline value (96 +/- 2 mm Hg) within 10 minutes (pHa, 7.33 +/- 0.001); at 60 minutes (pHa, 7.21 +/- 0.02), PaO2 was 108 +/- 2 mm Hg. For the same pHa decrease, the PaO2 increase was less in group III. In group I, hypocalcemia caused PaO2 to progressively increase (from 95 +/- 2 mm Hg to 104 +/- 3 mm Hg), which correlated (r = -0.66) significantly with a decrease in systolic blood pressure (from 156 +/- 9 mm Hg to 118 +/- 10 mm Hg). Parathyroidectomy did not alter PaO2 values.

CONCLUSIONS AND CLINICAL RELEVANCE

Induction of hypocalcemia and metabolic acidosis each increased PaO2 in anesthetized dogs, whereas acidosis-induced hypercalcemia attenuated that increase. In anesthetized dogs, development of metabolic acidosis or hypocalcemia is likely to affect ventilatory control.

Calcium metabolism and cardiovascular function after spaceflight

To determine the influence of dietary calcium on spaceflight-induced alterations in calcium metabolism and blood pressure (BP), 9-wk-old spontaneously hypertensive rats, fed either high- (2%) or low-calcium (0.02%) diets, were flown on an 18-day shuttle flight. On landing, flight animals had increased ionized calcium (P < 0.001), elevated parathyroid hormone levels (P < 0.001), reduced calcitonin levels (P < 0.05), unchanged 1,25(OH)(2)D(3) levels, and elevated skull (P < 0.01) and reduced femur bone mineral density. Basal and thrombin-stimulated platelet free calcium (intracellular calcium concentration) were also reduced (P < 0.05). There was a tendency for indirect systolic BP to be reduced in conscious flight animals (P = 0.057). However, mean arterial pressure was elevated (P < 0.001) after anesthesia. Dietary calcium altered all aspects of calcium metabolism (P < 0.001), as well as BP (P < 0.001), but the only interaction with flight was a relatively greater increase in ionized calcium in flight animals fed low- compared with high-calcium diets (P < 0.05). The results indicate that 1) flight-induced disruptions of calcium metabolism are relatively impervious to dietary calcium in the short term, 2) increased ionized calcium did not normalize low-calcium-induced elevations of BP, and 3) parathyroid hormone was paradoxically increased in the high-calcium-fed flight animals after landing.

Dietary calcium and blood pressure in experimental models of hypertension. A review

More than 80 studies have reported lowered blood pressure after dietary calcium enrichment in experimental models of hypertension. The evidence presented here suggests that dietary calcium may act concurrently through a number of physiological mechanisms to influence blood pressure. The importance of any given mechanism may vary depending on the experimental model under consideration. Supplemental dietary calcium is associated with reduced membrane permeability, increased Ca(2+)-ATPase and Na,K-ATPase, and reduced intracellular calcium. These results suggest that supplemental calcium may limit calcium influx into the cell and improve the ability of the VSMC to extrude calcium. This could be a direct effect of calcium on the VSMC or an indirect effect mediated hormonally. The calcium-regulating hormones have all been found to have vasoactive properties and therefore may influence blood pressure. Furthermore, CGRP and the proposed parathyroid hypertensive factor are both vasoactive substances that are responsive to dietary calcium. Therefore, diet-induced variations in calcium-regulating hormones may influence blood pressure. Modulation of the sympathetic nervous system is another important way that dietary calcium can influence blood pressure. There is evidence of altered norepinephrine levels in the hypothalamus as a consequence of manipulations of dietary calcium as well as changes in central sympathetic nervous system outflow. Dietary calcium has also been shown to specifically modify alpha 1-adrenergic receptor activity in the periphery. In some experimental models of hypertension, dietary calcium may alter blood pressure by changing the metabolism of other electrolytes. For example, the ability of calcium to prevent sodium chloride-induced elevations in blood pressure may be attributed to natriuresis. However, natriuresis does not account for all of the interactive effects of calcium and sodium chloride on blood pressure. Sodium chloride-induced hypertension may be due in part to calcium wasting and subsequent elevation of calcium-regulating hormones. Chloride is an important mediator of this effect because it appears that sodium does not cause calcium wasting when it is not combined with chloride. More attention to the central nervous system effects of dietary calcium is needed. Not only can calcium itself influence neural function, but many of the calcium-regulating hormones appear to affect the central nervous system. The influence of calcium and calcium-regulating hormones on central nervous system activity may have important implications for blood pressure regulation and also may extend to other aspects of physiology and behavior.