Summary
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1.
The effect of phentolamine on the response properties of insect mechanoreceptors and on the conduction in their axons was examined using electrophysiological techniques.
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2.
Phentolamine blocked conduction of action potentials along axons, an effect which exhibited 3 characteristics typical of local anesthetics: the effect was frequency-dependent, reversible and varied for nerves with different diameters.
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3.
The concentration of phentolamine required to block axonal conduction (1–2×10−3 M) was significantly higher than that required to abolish the response of receptors to mechanical stimulation (3–5×10−4 M).
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4.
All mechanoreceptors that were examined in Locusta migratoria and Periplaneta americana were inactivated by phentolamine (Table 1). The type I receptors (chordotonal, campaniform and hair sensilla) were inactivated within 5–15 min following phentolamine application. The only type II receptor examined (forewing stretch-receptor) underwent a phase of repetitive discharge before being inactivated.
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5.
Tolazoline and metoclopramide inactivated, like phentolamine, mechanoreceptors at lower concentrations than necessary to block axonal conduction. However, yohimbine and chlorpromazine inactivated mechanoreceptors and blocked axonal conduction at similar concentrations.
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6.
These findings suggest that phentolamine affects sense-organ specific ionic processes that are more sensitive to the drug than the ionic processes along the axons.
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Ramirez, J.M., Pearson, K.G. Local anesthetic action of phentolamine on insect mechanoreceptors. J Comp Physiol A 167, 475–483 (1990). https://doi.org/10.1007/BF00190818
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DOI: https://doi.org/10.1007/BF00190818