Extracellular calcium level is crucial for aortic contractile response in pregnant rat

Studies of vascular reactivity during late pregnancy were performed to investigate the previously described hyporeactivity to vasopressors. Two groups of seven control (nonpregnant) and seven late pregnant (day 20) Wistar rats were used. Rubbed (E-) segments from thoracic aorta were studied for contractile studies in a Krebs solution containing either 1.25 or 2.50 mmol/L Ca concentration. Norepinephrine (NE; 10-9-3 x 10-5) or depolarization induced (KCl, 100 mmol/L) contractions are given as mN/mm2. With 2.50 mmol/L Ca, the maximal contraction to NE in E-segments is decreased in late pregnant rats compared with nonpregnant rats. However, this difference disappears when calcium concentration is set to 1.25 mmol/L Ca, the physiological value for free calcium concentration in extracellular fluid. For the contraction induced by opening of voltage operated calcium channels (KCl depolarization), a decreased maximal tension is also obtained in the pregnant rats compared with nonpregnant only with 2.50 mmol/L Ca concentration. It appears that aortic response to vasoconstrictors is modulated by extracellular calcium concentration in a different way between pregnant and nonpregnant rats.