Resolution and pharmacological analysis of the voltage-dependent calcium channels of Drosophila larval muscles

Abstract

Voltage-dependent calcium channels play a role in many cellular phenomena. Very little is known about Ca2+ channels in Drosophila, especially those in muscles. Existing literature on neuronal Ca2+ channels of Drosophila suggests that their pharmacology may be distinct from that of vertebrate Ca2+ channels. This raises questions on the pharmacology and diversity of Ca2+ channels in Drosophila muscles. Here we show that the Ca2+ channel current in the body-wall muscles of Drosophila larvae consists of two main components. One component is sensitive to 1,4-dihydropyridines and diltiazem, which block vertebrate L-type Ca2+ channels. The second component is sensitive to amiloride, which blocks vertebrate T-type Ca2+ channels. In contrast to Drosophila brain membrane preparations in which a majority of the Ca2+ channels are phenylalkylamine-sensitive but dihydropyridine- insensitive, the major current in the muscles was dihydropyridine-sensitive but relatively less sensitive to verapamil. This might indicate an underlying tissue specific distribution of distinct subtypes of dihydropyridlne/phenylalkylamine-sensitive Ca2+ channels in Drosophila. Low verapamil sensitivity of the dihydropyridine-sensitive current of Drosophila muscles also set it apart from the vertebrate L-type channels which are sensitive to 1,4-dihydropyridines, benzothiazepines as well as phenylalkylamines. The dihydropyridine-sensitive current in Drosophila muscles activated in a similar voltage range as the vertebrate L-type current. As with the vertebrate current, blockade by dihydropyridines was voltage dependent. Compared to the vertebrate T-type current, the amiloride- sensitive current in Drosophila muscles showed higher activation threshold as well as slower inactivation. These experiments provide the first clear resolution of a Drosophila Ca2+ current into two distinct components. With the previous resolution of the K+ current into four components, Drosophila larval muscles now provide one of the few preparations in which the whole cell current can be resolved completely into individual ionic currents. This will help in determining the role of individual currents in cellular excitability and other calcium related processes; in analyzing structure, function, and regulation of specific types of Ca2+ channels; as well as in understanding the molecular basis of calcium channel diversity.