Chronic head-down-tilt sleeping as physiological regulator of bone remodelling during diminished muscular activity in humans

OBJECTIVES

Head-down-tilt (HDT) sleeping with periodic fluid redistribution (PFR) assumes a significant importance by the possibility of regulating bone remodelling. We hypothesized that HDT sleeping with chronic PFR which expands fluid volume would contribute to and/or increase bone formation. Therefore, we studied the potential benefits of osteogenesis with HDT sleeping of chronic PFR during diminished muscular activity (hypokinesia; HK).

METHODS

Studies were conducted on 40 male healthy volunteers. They were divided into four groups: head-down-tilt sleeping control subjects (HDTSCS), head-down-tilt sleeping hypokinetic subjects (HDTSHS), active control subjects (ACS) and hypokinetic subjects (HKS). The iliac crest cancellous bone and trabecular bone volume and cortical thickness were measured during pre-experimental period of 390 days and experimental period of 360 days.

RESULTS

Iliac crest cancellous bone and trabecular bone volume and cortical thickness were increased (P<0·05) in the HDTSHS group compared to the HKS group. Iliac crest cancellous bone volume, cortical thickness and trabecular bone volume were reduced (P<0·05) in the HKS group compared to their pre-experimental values and the level in the other groups. In the HDTSCS group, iliac crest cancellous bone volume, cortical thickness and trabecular bone volume were increased much less than in the HDTSHS group. Bone mineral density was not affected in the ACS group compared to their pre-experimental values.

CONCLUSION

The current study shows the impact of chronic HDT sleeping with PFR on increases formation of bone demonstrating osteogenesis of bone during diminished muscular activity.

Chronic periodic fluid redistribution effect on muscle calcium in healthy subjects during prolonged hypokinesia

Studies have shown that chronic periodic fluid shifting upwards is not sensed as excessive fluid volume and excretion mechanisms are not activated. To determine if chronic periodic fluid and volume shifting upwards can affect muscle calcium (Ca(2+)) during hypokinesia (HK) we measured muscle Ca(2+) content, plasma Ca(2+) concentration, and Ca(2+) losses in urine and feces. Studies were conducted on 40 healthy male volunteers. They were divided into four equal groups: active control subjects (ACS), hypokinetic subjects (HKS), periodic fluid redistribution control subjects (PFRCS), and periodic fluid redistribution hypokinetic subjects (PFRHS). Plasma Ca(2+) level decreased (p < 0.05) in Ca(2+) repleted muscle, muscle Ca(2+) level increased (p < 0.05), and Ca(2+) losses in urine and feces decreased (p < 0.05) in the PFRHS group compared with the HKS group. Plasma Ca(2+) level increased (p < 0.05) in Ca(2+) deficient muscle, muscle Ca(2+) level decreased (p < 0.05), and Ca(2+) losses in urine and feces increased (p < 0.05) in the HKS group compared with their pre-experimental levels and the values in their respective control groups (ACS and PFRCS). This study shows that the muscle Ca(2+) content increases and Ca(2+) excretion decreases, suggesting the clinical potential of chronic periodic fluid and volume redistribution in treatment of muscle Ca(2+) deficiency.

Effect of potassium and calcium loading on healthy subjects under hypokinesia and physical exercise with fluid and salt supplements

The objective of this investigation was to determine the acute responses to the electrolyte challenges under hypokinesia and physical exercise (PE) of different intensities with fluid and salt supplementation (FSS). The studies were performed on 12 physically healthy male volunteers aged 19-24 years under 364 days of hypokinesia (decreased number of steps per day) with a set of PE with FSS. The volunteers were divided into two equal groups. The first group was subjected to a set of intensive PE and the second group was submitted to a set of moderate PE. Both groups of subjects consumed daily water and salt supplements that aimed to increase the body hydration level. For simulation of the hypokinetic effect all subjects were kept under an average of 3000 steps per day. Functional tests with a potassium chloride (KCl) and calcium lactate (Cal) load were performed during the hypokinetic period of 364 days and the 60-day, prehypokinetic period that served as control, while both groups of subjects consumed daily calcium and potassium supplements. The concentration of electrolyte and hormone levels in the blood and their excretion rate in urine were determined. Renal excretion of calcium and potassium and the blood concentration thereof increased markedly in both groups of subjects. With the potassium chloride load tests the increased potassium excretion was accompanied by higher aldosterone and insulin blood levels, and with the calcium lactate load tests the increased calcium excretion was accompanied by a decreased parathyroid content in the blood. FSS and PE, regardless of intensity, failed to attenuate calcium and potassium losses. Additional intake of KCl and Cal also failed to normalize potassium and calcium abnormalities. It was concluded that during the KCl and Cal loading tests, the increased losses of potassium and calcium in the hypokinetic subjects were due to the inability of their bodies to retain these electrolytes, and that electrolyte abnormalities could not be reversed by PE or rehydration in individuals subjected to prolonged restriction of motor activity.