Nonsteady-State Three Compartment Tracer Kinetics: II. Sodium Flux Transients in the Toad Urinary Bladder in Response to Short Circuit

The theoretical approach presented in the previous paper provides an analytical method for determining the unidirectional, nonsteady-state fluxes in a three compartment system. Based on this a study was made of the sodium flux transients in the toad urinary bladder. A transient time-dependent state was generated by suddenly short-circuiting a bladder previously maintained in an open-circuited steady state. The sequence of experiments suggested by the theory provided the data required for the analysis. The results of these tracer experiments were consistent with the complex non-three compartmental structure of this tissue. As a result both of the inadequacy of the three compartment model in representing the tissue and of certain experimental difficulties, attempts at a quantitative solution were not entirely successful. Useful information was nevertheless obtained through a careful use of this model, and a qualitative analysis implied that the sodium influxes into the tissue at both of its surfaces are sensitive to changes in electrical potential while both effluxes are insensitive to this change. This suggests that both of the effluxes result from active processes while both influxes are associated with passive processes. The net transepithelial transport of sodium would then necessarily result from a more complex polarization than that proposed by Koefoed-Johnsen and Ussing.     

Noncomplementarity in base sequences between the cohesive ends of coliphages 186 and lambda and the formation of interlocked rings between the two DNA's

The half molecules of 186 DNA have been isolated by the Hg(II)–Cs₂SO₄ density gradient centrifugal ion technique. The buoyant densities of the two halves in CsCI at 25°C. are 1.713 and 1.709 g./cm.³, corresponding to GC contents of 54% and 50%, respectively. Similarly, 5-bromouracil labeled λ DNA halves were separated. The isolation of the four DNA halves made it possible to test for homology in base sequences between the cohesive ends of λ and those of 186. There was no indication of any significant homology in base sequences between the cohesive ends of the two DNA's, as indicated by the absence of a band with intermediate buoyant density in CsCI when either half of 186 DNA was annealed with either half of 5-bromouracil labeled λ DNA and then centrifuged. The lack of cohesion between the two DNA's made it possible to demonstrate unequivocally the formation of interlocked rings (catenanes) between the two DNA's. The existence of a dimeric catenane is evidenced by the formation of a species of intermediate buoyant density when 5-bromouracil labeled λ DNA is cyclized in the presence of cyclic 186 DNA of a relatively high concentration. The molecular weight of one DNA relative to the other can be calculated from the position of the dimeric catenane in a density gradient by using the method of Baldwin. The result was in complete agreement with our previous measurements from the sedimentation coefficients and by electron microscopy. The probability of dimeric catenane formation when one DNA is cyclized in the presence of another DNA is discussed. The experimental results agree with the theoretical expectation.