Noradrenaline Hyperpolarization and Depolarization in Cat Vesical Parasympathetic Neurones

Responses to noradrenaline (NA) applied by superfusion, ionophoresis or pressure pulse were analysed using conventional intracellular recording and voltage-clamp methods in cat vesical parasympathetic ganglia. NA (1 microM) hyperpolarized 60% of the neurones, depolarized 25%, and produced a biphasic potential, which comprised a membrane hyperpolarization followed by a membrane depolarization, in 10%. About 5% of the neurones did not respond to NA. The NA hyperpolarization was blocked by yohimbine (1 microM), an alpha 2-adrenoceptor antagonist, whereas the NA depolarization was blocked by prazosin (0.1-1 microM), an alpha 1-adrenoceptor antagonist. These data indicated that the NA hyperpolarization was mediated through alpha 2-adrenoceptors and the NA depolarization through alpha 1-adrenoceptors. The NA hyperpolarization was accompanied by an increase in conductance, while the NA depolarization was associated with a decrease in conductance measured under manual-clamp conditions. Similar conductance changes were observed under voltage clamp. NA hyperpolarizations became smaller as the membrane was hyperpolarized and reversed polarity beyond -100 mV. NA depolarizations also became smaller at hyperpolarized membrane potentials and reversed polarity around -90 mV. The NA responses were enhanced in low-K media and depressed in high-K Krebs solution. The NA hyperpolarization was blocked by the Ca antagonists, Cd, Mn and Co. Intracellular injection of EGTA caused a slowly developing, progressive block of the NA hyperpolarization. The NA depolarization was not affected by low Ca concentrations, Ca antagonists or intracellular injection of EGTA. In some neurones the NA depolarization was unmasked in solutions containing Ca antagonists and after intracellular EGTA injection. The NA hyperpolarization was depressed by intracellular injection and extracellular superfusion of Cs but not by TEA. Ba (10-100 microM) depressed the NA hyperpolarization by 30%. The NA depolarization persisted in the presence of muscarine (10 microM) and was not blocked by Cs or TEA but was depressed 70% by Ba (10 microM). These data are consistent with the hypotheses that alpha 2-adrenoceptor activation produces a membrane hyperpolarization that is mediated through a Ca-dependent K conductance, and that alpha 1-adrenoceptor activation produces a membrane depolarization through closure of a voltage-insensitive K channel.