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Laboratoire de Physiologie Cellulaire Cardiaque, Université Paris-Sud Bât 443, Orsay, France.

1. This study was performed to investigate the possible presence and role of the creatine kinase (CK) system in the contraction and relaxation of skinned guinea-pig uterus as well as the changes of the CK system during gestation. Experiments were performed on isolated longitudinal fibres of gravid and non-gravid myometrium. 2. Total CK activity increased from 74 +/- 11 to 196 +/- 39 IU (g wet wt)-1 during gestation. 3. The four isoenzymes of CK: muscle (MM), muscle-brain (MB), brain (BB) and mitochondrial (mt-CK) were found in myometrium. MM, MB and BB isoenzymes represented respectively 20.3 +/- 2.6, 10.3 +/- 4.4 and 72.7 +/- 2.2% of total activity. The distribution of isoenzymes did not significantly change with gestation, the contribution of mt-CK increasing from trace to 5% of total activity. 4. BB-CK was specifically bound to Triton X-100-skinned fibres with the non-gravid uterus containing 6.7 +/- 1.9 IU (g wet wt)-1 and the gravid uterus containing 44 +/- 13 IU (g wet wt)-1. 5. Active tension of Triton X-100-treated fibres increased from 6.06 +/- 0.68 to 19.3 +/- 1.9 mM mm-2 during gestation. 6. Submaximal tension (43.3 +/- 4.4% of maximal tension) can be developed in the absence of ATP and in the presence of 12 mM phosphocreatine (PCr) and 250 microM MgADP from endogenous CK in non-gravid uterine fibres while the gravid uterus was able to generate 65.4 +/- 3.9% of maximal tension via the CK system. 7. The endogenous CK system was able to relax the skinned fibres from high-tension rigor conditions by 47.3 +/- 4.2% of total relaxation in non-gravid fibres and 60.6 +/- 3.2% of total relaxation in gravid fibres. 8. Non-gravid and gravid uteri both contained mt-CK of 17.5 +/- 8.4 and 140 +/- 22 micrograms (g wet wt)-1 respectively as determined with antibodies against mt-CK. 9. Oxygen consumption was studied in fibres where the plasmalemma was solubilized with 50 micrograms ml-1 saponin. Maximal respiration was increased from 0.91 +/- 0.05 to 2.61 +/- 0.16 mumol oxygen min-1 (g dry wt)-1 in the gravid uterine fibres. However, creatine did not stimulate respiration in the uterine fibres treated with saponin. 10. It is concluded that the CK system undergoes qualitative as well as quantitative changes during gestation. BB-CK is specifically localized in the myofilaments and mt-CK is present in the uterine mitochondria.(ABSTRACT TRUNCATED AT 400 WORDS)