Insulin-stimulated sodium transport in toad urinary bladder

Area of Science:

• Comparative Physiology
• Endocrinology
• Renal Physiology

Background:

• Insulin is known to regulate glucose metabolism in mammals.
• Its role in ion transport, particularly in non-mammalian vertebrates, is less understood.
• The toad urinary bladder is a model system for studying active sodium transport.

Purpose of the Study:

• To investigate the effect of mammalian and teleost insulins on active sodium transport in the toad urinary bladder.
• To determine the concentration dependence and time course of insulin's action.
• To elucidate the mechanism of insulin-mediated sodium transport stimulation.

Main Methods:

• Measurement of short-circuit current (SCC) in isolated toad urinary bladders.
• Application of mammalian (porcine, bovine) and teleost insulins at subnanomolar concentrations.
• Assessment of insulin specificity using proinsulin and cross-linked insulin derivatives.
• Investigation of the role of protein synthesis using inhibitors (actinomycin D, puromycin, cycloheximide, tunicamycin).

Main Results:

• Subnanomolar concentrations of mammalian and teleost insulins rapidly increased SCC within 15 minutes, with effects persisting for hours.
• Porcine proinsulin and a modified bovine insulin were less effective, suggesting insulin receptor specificity.
• Initial stimulation was independent of protein synthesis, but sustained stimulation was blocked by inhibitors of transcription and translation, indicating a requirement for new protein synthesis.

Conclusions:

• Insulin effectively stimulates active sodium transport in the toad urinary bladder via a receptor-mediated mechanism.
• Sustained stimulation of sodium transport by insulin requires de novo protein synthesis.
• These findings reveal a conserved role for insulin in regulating ion transport across species.