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1. The velocity of unloaded shortening (V0), the myofibrillar ATPase activity and the immunoreactivity to two monoclonal antibodies (A1 and A2) that were raised against the myosin heavy chains were studied in single fibres of the anterior tibialis muscle of Rana temporaria. V0 was recorded for the fibre as a whole using the slack‐test method. Myofibrillar ATPase activity was determined by means of a quantitative histochemical technique. 2. A highly significant, direct relationship was found to exist between V0 and the myofibrillar ATPase activity recorded in the same single fibres. Both V0 and the myofibrillar ATPase activity changed in proportion to the cross‐sectional area of the fibres. 3. Muscle fibres that had first been characterized with respect to V0 and myofibrillar ATPase activity were exposed to monoclonal antibodies A1 and A2. Thin fibres, having relatively low V0 and low myofibrillar ATPase activity, reacted preferentially with A1. Thick fibres, on the other hand, exhibiting relatively high V0 and high myofibrillar ATPase activity, were preferentially stained by A2. A third category of fibres reacted with both A1 and A2. The results support the view that the variability in shortening velocity and myofibrillar ATPase activity that exists among twitch fibres in frog skeletal muscle is based on differences in myosin heavy‐chain composition. 4. Attempts were made to elucidate further the previous observation (Edman, Reggiani & te Kronnie, 1985) that the velocity of unloaded shortening (V0) differs along the length of individual muscle fibres. To this end discrete segments (0.5‐0.7 mm in length) of intact fibres were delineated by opaque markers of hair that were placed on the fibre surface. The change in length between two adjacent markers (one segment) was recorded photo‐electrically while the fibre was released to shorten against a very small load between 2.2 and 2.0 micron sarcomere lengths. In the majority of fibres (eight out of eleven preparations), V0 and myofibrillar ATPase activity exhibited similar patterns of variation along the fibre. Pooled data from thirty‐three segments of twelve fibres showed a positive correlation between V0 and myofibrillar ATPase activity (P less than 0.05). 5. The possibility was explored that the myosin isoform composition might vary along the length of an individual muscle fibre. For this purpose bundles of fibres were cross‐sectioned at 0.5‐1 mm intervals along their entire length and the reactivity to monoclonal antibody A2 was tested at each location.(ABSTRACT TRUNCATED AT 400 WORDS)

The velocity of 'unloaded' shortening (V0) and the force‐velocity relation were studied during fused tetani (0.5‐2.0 degrees C) in short successive segments along the entire length of single fibres isolated from the tibialis anterior muscle of Rana temporaria. The segments were defined by opaque markers of hair that were placed on the fibre surface, 0.5‐0.8 mm apart, from one tendon insertion to the other. The change in distance between two adjacent markers (one segment) was monitored by means of a photoelectric recording system, while the fibre was released to shorten isotonically between 2.2 and 2.0 micron sarcomere lengths. The accuracy of the V0 measurement was better than 4% in all parts of the fibre. V0 varied along the length of the fibre, each fibre having a unique velocity pattern that remained constant throughout the experiment. The difference between the highest and lowest values of V0 within the fibre varied between 11 and 45% of the fibre mean in thirty‐two preparations (mean difference 23 +/‐ 2%, S.E. of mean). An attempt was made to relate the V0 pattern to the fibre's orientation in the body in fourteen complete experiments. The highest values of V0 were obtained near the proximal end of the fibre, and there was a clear trend for V0 to assume lower values towards the distal end. The V0 pattern along the fibre did not correlate with the segments' capacities to produce force nor with the passive viscoelastic properties of the segments. Force‐velocity data obtained from individual segments provided a good fit to Hill's (1938) hyperbolic equation at loads less than 80% of the measured tetanic force. The curvature of the force‐velocity relation, defined by alpha/P0 in Hill's equation (P0 being the isometric force calculated from the hyperbolic function) varied between 0.09 and 0.46 in sixteen segments of six different fibres. V0 was inversely related to alpha/P0 according to the following regression: V0 = 3.21 ‐ 3.22. (alpha/P0), correlation coefficient, 0.72; P less than 0.005. No clear correlation between V0 and alpha/P0 existed at the whole‐fibre level. The results support the view that the kinetic properties of the myofilament system differ from one region to another along the length of a muscle fibre.

Several polycations added to the luminal solution were found to inhibit the vasopressin (ADH)-induced water flow in toad urinary bladder but not the ADH-induced increase in sodium transport or in urea permeability. Ultrastructural studies were conducted to evaluate the uptake of cationized ferritin. It was found that endocytosis of cationized ferritin by luminal cells was strikingly enhanced on exposure to ADH; this increased endocytosis was concomitant with inhibition of transepithelial ADH-induced water flow. Various maneuvers preventing endocytosis were also found to counteract the polycation-induced inhibition of the ADH effect. It is suggested that polycations are endocytosed in vesicles whose walls contain the water channels but not the urea or sodium channels.

Summary.Blastocysts were flushed from both uterine horns of 10 gilts on Day 11 of pregnancy. In these gilts 15·1 ± 2·3 (mean ± s.d.) corpora lutea were present and 12·7 ± 3·1 spherical blastocysts were recovered. In all the gilts variation in blastocyst diameter was observed. Aromatase activity was measurable in 118 of 121 examined blastocysts and varied from 0·005 to 19·330 pmol [1β-3H]androstenedione converted into 3H2O in 20 min (mean 1·31). This variation in aromatase activity reflected a difference between and within gilts. Of the total variation between all blastocysts, 67% was due to differences between gilts. A positive exponential relationship existed between blastocyst diameter and aromatase activity, but this relationship was different between gilts (P< 0·0001). The observed variation in aromatase activity may be caused by differences in germ layer differentiation of the blastocysts.Keywords:pig; blastocyst; diameter; aromatase

Summary.The embryonic ectoderm of the pig differentiated and became part of the outer barrier of the blastocyst (earlier formed by the trophectoderm alone) before shedding of the overlying polar trophectoderm around Day 10, thus securing the integrity of the rapidly expanding blastocyst. Ferritin, added to the medium of the blastocyst, was taken up rapidly by trophectoderm cells, but did not reach the blastocoele, and consequently no tracer was found within hypoblast cells. Embryonic ectoderm cells did not absorb the macromolecule, before or after loss of the polar trophectoderm. When ferritin was injected into the blastocoele, trophectoderm, hypoblast and embryoblast cells all absorbed the tracer. At Day 11, blastocyst diameter and embryoblast cell number varied widely and were hardly correlated. We suggest that embryoblast development may be a more reliable indicator for the developmental stage of a blastocyst than its diameter, which may merely be an indication of the viability of the trophoblast.Keywords:pig; blastocyst; ultrastructure; developmental variation