Freeze-dried skinned cardiac and skeletal muscle preparations of the rabbit were immersed in Ca2+-containing solutions with different concentrations of caffeine. The relation between the negative logarithm of the Ca2+ concentration (pCa) and normalized developed force was studied. The exact position of these Ca2+-sensitivity functions proved to be dependent on both the sarcomere length (monitored by means of laser diffraction) and caffeine concentration. High concentrations of caffeine induce a reversible fall in tension, particularly at low binding site saturation (low pCa) and long sarcomere lengths. At a concentration of 10 mM caffeine, the sarcomere length dependency of the Ca2+-sensitivity curves is markedly reduced for the rising part of the curve. Only the depressive effect of caffeine at high pCa remains. A possible mechanism of caffeine action is discussed.

Skinned cardiac and skeletal muscle freeze-dried preparations were activated in solutions strongly buffered for Ca2+. The response of single skeletal muscle fibres or thin strips of papillary muscle was investigated in relation to changes in Ca content of the perfusate. Sarcomere length was set and controlled during the experiments. The relation between the negative logarithm of the Ca concentration, the pCa, and the normalized developed force proved to be sigmoidal. The exact position of these curves proved to be dependent upon both sarcomere length and the distance between the filaments. The latter was shown by means of osmotic compression of the fibres using dextran. As a consequence of these observations, it was concluded that the length-tension relation is dependent upon the actual Ca concentration. The results are discussed in terms of cross-bridge interaction.

SUMMARY A technique has been developed for the autoradiographic localization of steroids in testes at the light and possibly electron microscope level. During processing of the tissue there is no contact of tissue with water up to the moment of photographic development of the autoradiographs. Tritium labelled steroids have been introduced into the testis through perfusion of the isolated organ. Small tissue samples were rapidly frozen, freeze dried and fixed with osmium vapour. The fixed tissue was embedded in Epon and thin sections could be cut from this material on an ultra‐cryotome without the aid of floatation liquids. Results with light microscopic autoradiography indicate the potential of this technique. This procedure which avoids any contact of the tissue with water offers good prospects for autoradiography of steroids and other soluble compounds at the electron microscope level. A comparison is made with other proposed techniques for steroid autoradiography.

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