Biopsies will be obtained from pregnant women undergoing planned caesarean section at term. The biopsies will divided into 3 strips that will be stored overnight in chilled NaCl 0.9%. The following morning each strip will be mounted isometrically with one end fixed to a holder and the other end to a force transducer. One strip will be maintained in a solution containing 140mmol/L Na,acting as control, whereas the other two strips will be placed in fluids with diminishing Na concentration, from 140 mmol/L, to 130 mmol/L and 120 mmol/L. Contractility will be measured with force transducers and a polygraph