Phosphodiesterase activities for cyclic nucleotides in nerve endings from the bovine posterior pituitary gland

Nonselective cation conductance activated by muscarinic and purinergic receptors in rat spiral ganglion neurons

The present study characterizes the ionic conductances activated by acetylcholine (ACh) and ATP, two candidate neuromodulators, in isolated spiral ganglion neurons (SGNs). Brief application (1 s) of ACh evoked in a dose-dependent manner (EC(50) = 4.1 microM) a reversible inward current with a long latency (average 1.3 s), at holding potential (V(h)) = -50 mV. This current was reversibly blocked by atropine and mimicked by muscarine. Application of ATP also evoked a reversible inward current at V(h) = -50 mV, but the current showed two components. A fast component with a short latency was largely reduced when N-methyl-D-glucamine (NMDG) replaced extracellular sodium, implying a P2X-like ionotropic conductance. The second component had a longer latency (average 1.1 s) and was presumably activated by metabotropic P2Y-like receptors. The second component of ATP-evoked current shared similar characteristics with the responses evoked by ACh: the current reversed near 0 mV, displayed inward rectification, could be carried by NMDG, and was insensitive to extracellular and intracellular calcium. This ACh-/ATP-evoked conductance was reversibly inhibited by preapplication of ionomycin. These results suggest that muscarinic receptors and purinergic metabotropic receptors activate a similar large nonselective cation conductance via a common intracellular pathway in SGNs, a candidate mechanism to regulate neuronal excitability of SGNs.

Cyclic nucleotide phosphodiesterase activity in bovine brain coated vesicles

Cyclic AMP phosphodiesterase activity in bovine brain coated vesicles displayed a Km of approximately 22 microM for cyclic AMP, a Vmax of 3.2 nmol/min/mg protein, and a Hill coefficient of 1.5, suggesting positive cooperativity. The enzyme activity was stimulated by cyclic GMP with maximal indexes of stimulation ranging between 40 and 300%. Both basal and stimulated phosphodiesterase activities were immunotitrated with polyclonal antibodies against clathrin attached to heat-inactivated, formaldehyde-fixed Staphylococcus aureus cells. The main form of phosphodiesterase activity present in the immunoprecipitated brain coated vesicle preparation also is stimulated by cyclic GMP. The allosteric behavior was modulated by cyclic GMP. All of these properties are typical of type II or cyclic GMP-sensitive phosphodiesterases in addition to their calcium and calmodulin independence. Competition experiments with a series of phosphodiesterase inhibitors, papaverine, 1-methyl-3-isobutylxanthine, and theophylline, showed inhibition of cyclic AMP hydrolysis. Trifluoperazine was inactive at the highest concentration used, 100 microM. These compounds also inhibited the cyclic GMP-stimulated cyclic AMP hydrolysis with trifluoperazine practically inactive. At 5 microM cyclic AMP none of the inhibitors was seen to stimulate the cyclic AMP hydrolytic activity. The presence of an enzyme for the breakdown of cyclic nucleotides in brain coated vesicles may suggest a role for these second messengers in the in vivo functions of this organelle.

Forskolin: its effects on potassium-evoked release of vasopressin from the rat neurohypophysis

The effect of forskolin, added either before or 5 min after the onset of potassium-evoked release of vasopressin from isolated neurointermediate lobes of the rat has been investigated. A low concentration of forskolin (1 microM), added before stimulation, enhanced the potassium-evoked release of vasopressin throughout two successive 5 min periods of stimulation. Higher concentrations of forskolin (10-80 microM) produced no effect on the potassium-evoked release of hormone during the first 5 min of stimulation, but inhibited release during the second 5 min period. When added 5 min after the onset of potassium stimulation, forskolin (1-80 microM) reduced the amount of vasopressin released during the remaining 5 min of stimulation. Forskolin produced a concentration-dependent increase in cyclic AMP during both the control and potassium stimulation periods. The amount of cyclic AMP generated by forskolin during potassium stimulation was less than that produced during the corresponding control periods.