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Regulation of the cell magnesium in vascular smooth muscle
Abstract:
1. The relation between [Mg](o) and the Mg content of the rat tail artery incubated in Ca-free solutions was studied.2. Variation of [Mg](o) in Ca-free Krebs solution between 0.6 and 2.4 mm did not affect the cell Mg. In ATP-depleted arteries, a sizeable fraction of the cell Mg was found to be proportional to [Mg](o).3. An even larger fraction of the cell Mg became proportional to [Mg](o) in metabolically active arteries in which the transmembrane gradient of Na had been dissipated as a result of inhibition of the Na pump. In contrast to the extracellular Mg, the fraction responded to a change in [Mg](o) at a very slow rate. The size of the fraction was reflected in net uptake of Mg if [Mg](o) was higher than 1.2 mm. The rate of the uptake was lowered markedly by external Ca. All the Mg taken up by the cells was extruded again after restoration of the Na gradient.4. The uptake of Mg in Ca-free, ouabain-containing solution took place even if the concomitant swelling of cells was prevented by substitution of isethionate for external Cl.5. Under steady-state conditions and at const. [Mg](o) and [Na](o), [Mg](i) increased with increasing [Na](i).6. The results are consistent with the hypothesis that the outwardly directed Mg pump in rat vascular smooth muscle utilizes the energy released in the course of spontaneous influx of Na.
Insights
Extracellular magnesium ([Mg](o)) influences intracellular magnesium ([Mg](i)) in rat tail artery smooth muscle, particularly when the sodium-potassium pump is inhibited. Magnesium uptake is regulated by external magnesium concentration and external calcium.
Area of Science:
- Vascular smooth muscle physiology
- Cellular magnesium transport
- Ion gradients in arterial tissue
Background:
- Cellular magnesium homeostasis is crucial for vascular function.
- The relationship between extracellular magnesium ([Mg](o)) and intracellular magnesium ([Mg](i)) in vascular smooth muscle is not fully understood.
- Previous studies suggest complex interactions between magnesium, sodium, and calcium in vascular cells.
Purpose of the Study:
- To investigate the relationship between extracellular magnesium concentration ([Mg](o)) and intracellular magnesium ([Mg](i)) in rat tail artery smooth muscle.
- To determine the influence of metabolic state and ion gradients on magnesium handling.
- To elucidate the mechanism of magnesium transport in vascular smooth muscle cells.
Main Methods:
- Incubation of rat tail artery segments in modified Krebs solutions with varying extracellular magnesium concentrations ([Mg](o)) and absence of extracellular calcium.
- Manipulation of cellular ATP levels and inhibition of the Na pump (using ouabain) to alter cellular metabolic state and ion gradients.
- Measurement of intracellular magnesium ([Mg](i)) content under different experimental conditions.
- Assessment of magnesium uptake and extrusion dynamics.
- Investigation of the role of extracellular calcium and chloride substitution on magnesium transport.
Main Results:
- In Ca-free solutions, varying [Mg](o) between 0.6 and 2.4 mM did not alter [Mg](i) in ATP-depleted arteries, but a fraction of [Mg](i) became proportional to [Mg](o).
- In metabolically active arteries with dissipated Na gradients, a larger fraction of [Mg](i) correlated with [Mg](o) and responded slowly to changes in [Mg](o).
- Net Mg uptake occurred when [Mg](o) exceeded 1.2 mM, was slowed by external Ca, and all taken-up Mg was extruded upon Na gradient restoration. Mg uptake occurred even without cell swelling in Ca-free, ouabain-containing solution.
- [Mg](i) increased with increasing intracellular sodium ([Na](i)) under steady-state conditions with constant [Mg](o) and [Na](o).
Conclusions:
- A fraction of intracellular magnesium ([Mg](i)) in rat vascular smooth muscle is dependent on extracellular magnesium ([Mg](o)), particularly under conditions of impaired Na pump activity.
- The results support a model where an outwardly directed Mg pump utilizes the energy from spontaneous Na influx.
- External calcium significantly influences the rate of magnesium uptake into vascular smooth muscle cells.