Physiological Action of Drugs and Insecticides on Insects

Abstract

The action of certain drugs and insecticides on the frequency of the beat of the isolated cockroach heart, immersed in an aerated physiological solution at 29°C., have been studied. The frequency of the heart-beat is counted by eye and the drugs applied in the physiological solution. Acetylcholine at a concentration of 5 × 10-8to 10-5causes a sustained acceleration of the heart-beat which is blocked by atropine (5 × 10-5) and is unaltered by hexamethonium (5 × 10-5), prolonged exposure to nicotine (5 × 1O-5) or ergotamine (3 × 10-6). In this range of concentration ACH has a “muscarine-like” action.Adrenaline (3 × 10-6to 5 × 10-8) also causes an acceleration of the heart-beat, which soon passes off. Nor-adrenaline has the same action. Nicotine, hexamethonium or atropine do not modify the action of adrenaline, but ergotamine antagonises it.Nicotine (5 × 10-5produces an acceleration of the heart-beat which declines with prolonged exposure until ultimately no effect is produced on reapplieation, but recovery occurs with sufficient washing. Hexamethonium antagonises the effects of low concentration of nicotine. It is concluded that nicotine acts on the cardiac ganglia. Since atropine substantially reduces the action of nicotine, it is suggested that the ganglia excited may be chiefly cholinergic.ACH in high concentration (10-3) in the presence of atropine (5 × 10-5) has a stimulant effect which is abolished by nicotine. It is inferred that under these conditions, ACH is stimulating ganglia which may be adrenergic.