THE EFFECT OF THYROID HORMONES ON THE FORCE OF CONTRACTION OF LEFT ATRIA OF GUINEA-PIGS IN UITRO

Alterations in developed contractile force over a wide range of frequencies of electrical stimulation (0.1 to 6 Hz) were investigated in guinea-pig isolated left atria from animals pretreated for varying lengths of time with L-thyroxine or carbimazole. The guinea-pigs were pretreated with either L-thyroxine (lOug, lOOug and Img/kg body weight/day) or with carbimazole (Img and 2mg/kg body weight/day) for various treatment durations (between 1 day and 8 weeks) in order to establish whether the onset of action of thyroxine and carbimazole were duration- and dose-dependent. At 37°C, thyroxine produced both negative and positive inotropic effects at low and high frequencies of stimulation, respectively. Carbimazole produced a negative inotropic effect at all frequencies studied. Lowering the temperature of the bathing fluid to 28°C, permitted thyroxine to show a positive inotropic effect at all frequencies of stimulation. The negative inotropic effect of carbimazole was similar at both 37°C and 28°C. When the experimental conditions were kept as near to the physiological as possible, thyroxine increased and carbimazole decreased the contractility of the guinea-pig myo- cardium. Both the stimulatory action and the adverse effects of thyroxine were dose- and duration dependent whereas the inhibitory effect of carbimazole was only slightly dose-dependent. Carbimazole added in vitro exerted a negative inotropic effect on the rested- state contraction of guinea-pig left atrial preparations, suggesting a direct action of carbimazole on the myocardium. Findings in this study do not support the suggestion that thyroxine and carbimazole treatments alter myocardial contractility via the autonomic nervous system or by altering the activity of the Na+, K'*',ATPase enzyme. Thyroxine probably increases sarcolemmal calcium-influx and calcium stores in the SR and carbimazole probably produces opposite effects.

Name dc:type.qualificationname

PhD

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

De Montfort University

Year dc:date.issued

1987

Author dc:creator

• YUSOFF, ZURAIDAH NOHD

Licence

• https://dora.dmu.ac.uk/bitstreams/79f95be0-dbc8-4a79-a7fc-81938a6bd9fa/download