Pearson, Hugh Anthony; (1990) Physiology and pharmacology of insect calcium channels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

n this study the physiology and pharmacology of insect Ca2+ channels has been investigated using two preparations. In a skeletal muscle preparation from larvae of the moth Plutella xylostella, all or none action potentials were studied in the presence of TEA. These action potentials were Ca2+-dependent as was shown by the decline in amplitude and maximum rate of rise of the action potential when the extracellular Ca2+ concentration was reduced, abolition in Ca2+-free saline and block by inorganic Ca2+ channel blocking agents. Furthermore, TTX had no effect on the action potential. Removal of Cl- ions from the external medium resulted in a decrease in the maximum rate of rise of the action potential, suggesting the involvement of inward Cl-currents. However, the Cl- channel blocker ethacrynic acid was without effect. The organic Ca2+ channel antagonists of all 3 classes had no effect on the action potential suggesting that L-type channels were not involved. Furthermore, Lambert-Eaton myasthenic syndrome IgG had no effect on the action potential suggesting antigenic differences to L-type calcium channels. Ryanodine greatly prolonged the action potential possibly due to decrease in release of Ca2+ from intracellular stores, while the antihelminthic agent avermectin, although causing changes in the passive properties of cells had no effect on the action potential indicating a lack of action on insect calcium channels. To investigate neuronal Ca2+ channels, cells from thoracic ganglia of the locust Schistocerca gregaria were studied under whole cell patch clamp. Using 10mM Ba2+ or Ca2+ as the charge carrier, large biphasic currents were stimulated by depolarisation from a holding potential of -80mV. These currents were unaffected by replacement of external Na+ with choline and thus were not carried by TTx-insensitive Na+ currents. The two current components (sustained and transient) showed the same steady state inactivation kinetics, but could be separated by 10μM Cd2+ which selectively blocked the sustained current. For the organic antagonists, verapamil inhibited both components of current while nitrendipine was without effect. Single channel recordings of Ca2+ channels proved to be difficult, the only channel seen appearing to have transient characteristics. These data suggest that the channels in both skeletal muscle and nerve of insects do not readily fit into the pharmacological classification of L-, N- and T-type Ca2+ channels used in many vertebrate systems. Finally, ryanodine, avermectin and the pyrethroids S-bioallethrin and deltamethrin were found to have no effects on Ca2+ channels in locust neurones.

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