The uptake of biogenic monoamines by the isolated auricle of the snail (Helix pomatia).

Abstract

—The uptake of [3H]5HT, [3H]dopamine, [3H]noradrenaline and [3H]octopamine into the auricle of Helix pomatia was studied. When tissues were incubated at 25°C in media containing radioactive amines, tissue:medium ratios of about 49:1, 14:1 and 5:1 for 5-HT, dopamine, noradrenaline, and octopamine respectively were obtained after a 20–30 min incubation time. Tissues incubated at 25°C in media containing radioactive amines for 20–30 mins showed that almost all (96%) the radioactivity was present as unchanged [3H]5-HT, [3H]dopamine, [3H]octopamine or [3H]noradrenaline. The high tissue:medium ratios for 5-HT and dopamine, but not for noradrenaline and octopamine, showed saturation kinetics which were dependent upon temperature and sodium ions. From the Lineweaver–Burk plots, two uptake mechanisms for 5-HT at 25°C were resolved; the high affinity uptake process having a Km1 value of 6.0 ± 10−8m and a Vm1 value of 0.115 nmol/g/min while the lower affinity process had a Km2 value of 1.04 ± 10−6m and a Vm2 value of 0.66nmol/g/min. At 0°C a single uptake mechanism for 5-HT occurred which gave a Km value of 5.02 ± 10−8m and a Vm value of 0.0165 nmol/g/min. In the case of dopamine, the Lineweaver–Burk plot at 25°C showed a single uptake process with values for Km and Vm of 1.55 ± 10−7m and 0.086 nmol/g/min respectively. This process did not function at 0°C. The effect of various agents and ions upon the accumulation processes for all amines was also studied, and the data indicate that the same neurons probably accumulate more than one amine type. It is concluded that 5-HT and dopamine uptake in the auricle is a mechanism for inactivating these substances at 25°C and that an uptake mechanism for 5-HT also functions at 0°C. The results are discussed from the point of view of 5-HT's being the cardioexcitatory substance in the snail heart.