Electrophysiological and cable parameters of perfused beetle malpighian tubules.

Isolated perfused Malpighian tubules of the desert beetle Onymacris plana (Coleoptera: Tenebrionidae) have been subjected to cable analysis under the following conditions: control, adenosine 3',5'-cyclic monophosphate (cAMP), corpora cardiaca homogenate (CCH), and high ambient K (130 mM). In addition, we investigated possible effects of perfusate composition on proximal transtubular potential (Vo) by reducing K, Na, or Cl or by adding ouabain, furosemide, or dinitrophenol. The effects of cAMP, CCH, and high K on Vo and cable parameters were consistent with increased fluid secretion, i.e., diminished input and core resistances and increased virtual short-circuit current, length constant, and luminal diameter. They differed in that CCH had variable effects on Vo and high K did not reduce transepithelial resistance. In terms of their effects on the parameters of a simple equivalent electrical circuit, the responses to cAMP, CCH, and a high ambient K concentration appear to be mediated by different mechanisms. Alterations in perfusate composition were almost without effect.

Aluminium excretion by the distal tubule of the pig kidney.

The renal tubular handling of aluminium was investigated, using the stop-flow method, in 8 pigs (2 controls, 2 'dose range', and 4 experimental). Aluminium was measured by graphite furnace atomic absorption spectroscopy. In 11 stop-flow studies in the 4 experimental pigs, intratubular aluminium concentrations, corrected for water reabsorption, peaked in the distal nephron. Interrupted stop-flow revealed that aluminium was excreted into the tubule at this site. The aluminium excretion site was situated at or close to the sites of maximal calcium and sodium reabsorption. Aluminium excretion occurs in the distal tubule of the pig kidney.

Transepithelial and intracellular potentials in isolated Malpighian tubules of tenebrionid beetle.

Malpighian tubules of Onymacris plana (Coleoptera: Tenebrionidae) have been isolated for measurement of transepithelial and intracellular potentials, before and during stimulation of fluid secretion. In a bathing medium resembling the hemolymph composition of the insect, the transepithelial potential (VT) was approximately 13 mV, lumen positive. VT was subject to drift and frequently showed super-imposed regular oscillations, which were apparently action potentials associated with contractions of muscle fibers running along the tubules. Although tubules of Onymacris are approximately 8 cm long, the basal membrane potential (Vb) did not vary with distance along the tubule, averaging -31 mV. Addition of adenosine 3',5'-cyclic monophosphate (cAMP) or diuretic hormone (DH) homogenate to the bathing medium had no effect on Vb, but opposing effects on VT: cAMP caused it to increase to 60 mV, whereas DH homogenate caused a rapid drop in VT to almost zero. Ion substitutions in the bathing medium showed that under control conditions beetle tubules possessed appreciable basal permeability to both K and Cl ions, with a 10-fold reduction in bath K concentration hyperpolarizing Vb by 54 mV. The basal K and Cl channels were partially blocked by barium and thiocyanate ions, respectively. Stimulation with cAMP increased the apical membrane potential (Va) and significantly reduced the Cl permeability of the basal membrane, whereas its Na permeability remained negligible.

Mesotocin and vasotocin, two neurohypophysial hormones in the ostrich, Struthio camelus.

Two neurohypophysial hormones have been isolated from an avian species, the ostrich, Struthio camelus. Both have been characterized by amino acid analysis and sequence determination. The data obtained suggest that the oxytocin-like hormone is [Ile8-oxytocin] (mesotocin) and the vasopressin-like hormone is [Ile3-vasopressin] (vasotocin). Bioactivity measurements based on urinary conductivity showed vasotocin to be about five times as active as mesotocin.

An iterative algebraic simulation of renal glomerular dynamics.

Current models of renal glomerular dynamics require the solution of multiple differential equations, or utilise network thermodynamics, to describe the changing pattern of transcapillary forces and flows along the length of the glomerular capillary. The iterative algebraic algorithm described here simplifies the simulation considerably. Implemented on a microcomputer, the simulation has a run-time of but a few seconds, while yielding results identical to those described previously.

Blood-volume expansion in the rat: natriuresis accompanied by a fall in filtration fraction.

1. We have attempted to confirm the existence of a natriuretic hormone released in response to acute expansion of blood volume. 2. Isolated kidneys, perfused with whole blood at constant pressure, were incorporated within an extracorporeal circulation in recipient rats. In six control experiments urine flow rate, renal blood flow, glomerular filtration rate, filtration fraction, and the fractional excretion of filtered sodium and water were measured for periods of up to 120-140 min thereafter. The same variables were measured in a further 12 experiments in which, after 63 +/- 11 min, the rats were volume expanded with equilibrated whole blood (15, 18 or 28 ml/kg body wt). 3. On average the controls revealed no change in any of the variables measured; volume expansion was followed by increased renal blood flow and fractional excretion of filtered sodium and water, while the filtration fraction fell. 4. In both the control and volume-expansion experiments, there were 12 instances in which the fractional excretion of filtered sodium increased; in 10 of these, including those experiments in which the natriuresis was most marked, there was a closely correlated fall in filtration fraction. 5. In all the experiments changes in the fractional excretion of filtered sodium and water varied in parallel. 6. We conclude that volume expansion (a) changes the concentration of some circulating vasoactive substance(s) and (b) results in natriuresis and diuresis consequent upon a fall in filtration fraction.

Re-evaluation of the 'contralateral' effects of sodium and potassium on sodium transport through toad skin.

1. Changes in the concentration of Na in the outer bathing solution, [Na]o, or of K in the inner bathing solution, [K]i, alter the electrical responses of the isolated toad skin to changes in ionic concentrations in the contralateral solutions. The mechanism(s) of these apparently contralateral effects remain(s) unknown. 2. The phenomenon has been investigated here in the isolated abdominal skin of Xenopus laevis. Each skin was exposed to multiple levels of [Na]o and [K]i, of between 5 and 112 m-mole 1.(-1) The p.d. and short-circuit current (s.c.c.) responses were analysed both in terms of kinetics and in terms of changes in the equivalent electrical circuit of the Na transport mechanism. 3. Kinetic analysis revealed that the relationship between [Na]o and s.c.c., at any level of [K]i, followed Michaelis-Menten kinetics. Increasing levels of [K]i reduced the s.c.c. response to changes in [Na]o, conforming with the algebraic descriptions of 'slope-parabolic competitive inhibition'. High levels of [Na]o (of 60-112 m-mole 1.(-1)) occasionally reduced the s.c.c. in a manner reminiscent of 'substrate inhibition'; this effect was independent of the level of [K]i. At high [K]i and low [Na]o, s.c.c was again often less than that predicted by Michaelis-Menten kinetics. 4. In terms of the equivalent electrical circuit, increasing [Na]o produced a fall in Rseries; in the presence of 'substrate inhibition', however, Rseries rose on increasing [Na]o; in either case, ENa and Rsh remained unchanged. Increasing [K]i lowered both ENa and Rsh; Rseries fell with modest increments in [K]i, but increased at higher levels of [K]i. 5. These results can be interpreted without invoking unknown contralateral effects. Thus the changes in s.c.c., as induced by changes in [Na]o or [K]i, are consistent with homolaterally mediated effects on an enzymic mechanism of transepithelial Na transport; the changes in p.d., given the [K]i-dependent changes in Rsh, are similarly explicable.

Lack of effect of blood-volume expansion on short-circuit current across an isolated toad skin incorporated in the circulation of a rat.

1. Evidence for the existence of 'natriuretic hormone' resides, in part, in the demonstration that blood volume expansion in the dog is followed by a transient fall in short-circuit current (SCC) across a frog skin incorporated within its circulation. 2. We have attempted to confirm this effect in the rat, with a toad skin (Xenopus laevis) incorporated within the circulation. The skins, bathed in whole rat blood, displayed low SCC; skins bathed in 'mammalian' Ringer solution displayed equally low SCC, but responded normally to pitressin or amiloride. 3. When volume expansion was induced in ten rats by infusion of equilibrated whole blood (28 ml/kg body weight) there was a brisk rise in systemic blood pressure, diuresis, natriuresis and kaliuresis. 4. This blood-volume expansion was without detectable effect on the SCC across the skins incorporated within the rats' circulations.

Resolution of parameters in the equivalent electrical circuit of the sodium transport mechanism across toad skin.

In amphibian epithelia, amiloride reduces net sodium transport by hindering the entry of sodium to the active transport mechanism, that is, by increasing the series resistance (Rser). Theoretically, therefore, analysis of amiloride-induced changes in potential differences and short-circuit current should yield numerical estimates of all the parameters in the equivalent electrical circuit of the sodium transport mechanism. The concept has been explored by analysis of such changes in toad skins (Xenopus laevis) bathed in hypotonic sulphate Ringer's, after exposure to varying doses of amiloride, or to amphotericin, dinitrophenol or Pitressin. The estimated values of Rser, of the electromotive force of the sodium pump (ENa), and of the shunt resistance (Rsh) were independent of the dose of amiloride employed. Skins bathed in hypotonic sulphate Ringer's exhibited a progressive rise in ENa. Amphotericin produced a fall in Rser, while dinitrophenol caused a fall in ENa; washout of the drugs reversed these effects. Pitressin produced a fall in both Rser and Rsh, with a rise in ENa. These results are in accord with earlier suggestions regarding the site(s) of action of these agents.

Peritubular pH and transtubular potentials in isolated perfused cortical collecting ducts of rabbit kidney.

1. Cortical collecting ducts were dissected from slices of rabbit kidney, then perfused in vitro.2. Transtubular electrical potentials were measured before and after abrupt changes in peritubular fluid pH.3. Variation in peritubular fluid pH, induced either by alteration in HCO(3) (-) concentration or in H(2)PO(4) (-)/HPO(4) (2-) ratio, produced biphasic responses in potential. Thus, reduction in pH caused an immediate fall, and then a prolonged and marked rise, in transtubular potential. The converse occurred on raising the pH.4. Variation in luminal fluid pH between pH 4.85 and pH 7.35 did not alter this pattern of response.5. In contrast to the above, reduction of peritubular fluid pH by elevation of P(CO2) produced either no effect or a decrease in transtubular potential.6. The transtubular potential of the cortical collecting duct appears to be a function of the pH gradient across some as yet unidentified part of the wall of the duct.