Electrochemical characteristics of ion secretion in malpighian tubules of the New Zealand alpine weta (Hemideina maori) |
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Authors: | Leader J P Neufeld D S |
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Institution: | Department of Physiology, P.O. Box 913, University of Otago, Dunedin, New Zealand |
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Abstract: | Characteristics of ion and fluid secretion were investigated in isolated Malpighian tubules of the New Zealand Alpine Weta (Hemideina maori). Fluid secretion by tubules in iso-osmotic saline (500mOsm) occurred at a rate of 15+/-3nlh(-1) and was enriched in K(+) (approx. 125mmoll(-1)) relative to the saline (10mmoll(-1)). Maximal fluid secretion (112nlh(-1)) during simultaneous exposure to hypo-osmolality and dibutyryl cAMP resulted in an 8.8x increase in the quantity of K(+) secreted, compared to only a 2.4x increase in Na(+) secretion. Measurements of intracellular ion activities and membrane potentials indicated that Na(+) and K(+) were transported against a strong electrochemical gradient across the apical surface, regardless of saline osmolality. On the basolateral surface, there was a large driving force for Na(+) entry, while K(+) was distributed near its equilibrium potential. Neither bumetanide nor ouabain in the bathing saline had a significant effect on fluid secretion, but Ba(2+) and amiloride decreased fluid secretion by 79 and 57%, respectively. The effect of Ba(2+) on fluid secretion was consistent with a high basolateral permeability to K(+), relative to Na(+) and Cl(-). These results indicate that the characteristics of fluid secretion in this primitive insect are largely conserved with characteristics reported for other insects. |
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