Suppressive responses to calcium and catecholamines in immobilization stress-loaded rats

1. Modulations of electrophysiological actions on the right atria of immobilization (restraint) stress-loaded rats by extracellular Ca2+ concentration, [Ca]0, catecholamines and some drugs were examined. 2. The serum concentration of corticosterone increased compared with native rats, and peptic ulcerization was observed. 3. The stress-loaded rats had a stronger contraction and a higher sinus rate. Increasing [Ca]0 (0.9-3.6 mM) enhanced the contractile force in native rats, whereas increasing [Ca]0 inhibited it in stress-loaded rats. The cycle length decreased with an increase in [Ca]0 in native rats more than in stress-loaded rats. 4. Enhancement induced by isoprenaline (0.1-10 microM) at normal [Ca]0 was more suppressive in stress-loaded hearts. At 0.1 to 10 microM, norepinephrine and phenylephrine significantly caused a negative inotropic effect in both native and stress-loaded hearts, and norepinephrine caused a positive chronotropic effect in native hearts but not in stress hearts. 5. Both taurine and nicotine had negative inotropic and chronotropic effects in native rats. However, the drugs enhanced the contractile force in stress rats. 6. These results indicate that immobilization stress decreases the sensitivities to [Ca]0, and increasing [Ca]0 easily elicits Ca2+ overload. Low sensitivities to sympathetic drugs also are produced by the stress loading, whereas taurine and nicotine may exhibit protective actions.