ABSTRACT
Voltage-gated calcium channels (Ca(v)) are tonically up-regulated via Ras/extracellular signal-regulated kinase (ERK) signalling in sensory neurones. However, the mechanisms underlying the specificity of cellular response to this pathway remain unclear. Neurotrophic factors are attractive candidates to be involved in this process as they are key regulators of ERK signalling and have important roles in neuronal survival, development and plasticity. Here, we report that in rat dorsal root ganglion neurones, endogenous nerve growth factor (NGF), glial derived neurotrophic factor (GDNF) and epidermal growth factor (EGF) are all involved in tonic ERK-dependent up-regulation of Ca(v) channels. Chronic (overnight) deprivation of growth factors inhibits total Ca(v) current according to developmental changes in expression of the cell surface receptors for NGF, GDNF and EGF. Whilst EGF specifically regulates transcriptional expression of Ca(v)s, NGF and GDNF also acutely modulate Ca(v) channels within a rapid ( approximately 10min) time-frame. These acute effects likely involve changes in the biophysical properties of Ca(v)s, including altered channel gating rather than changes in surface expression. Furthermore, NGF, GDNF and EGF differentially regulate specific populations of Ca(v)s. Thus, ERK-dependent regulation of Ca(v) activity provides an elegant and extremely flexible system with which to tailor calcium influx to discrete functional demands.
Subject(s)
Calcium Channels/metabolism , Extracellular Signal-Regulated MAP Kinases/metabolism , Ganglia, Spinal/metabolism , Intercellular Signaling Peptides and Proteins/metabolism , Ion Channel Gating/genetics , Neurons, Afferent/metabolism , Animals , Animals, Newborn , Calcium Channels/drug effects , Calcium Channels/genetics , Cell Membrane/drug effects , Cell Membrane/genetics , Cell Membrane/metabolism , Cells, Cultured , Epidermal Growth Factor/metabolism , Epidermal Growth Factor/pharmacology , Extracellular Signal-Regulated MAP Kinases/drug effects , Ganglia, Spinal/drug effects , Glial Cell Line-Derived Neurotrophic Factor/metabolism , Glial Cell Line-Derived Neurotrophic Factor/pharmacology , Intercellular Signaling Peptides and Proteins/pharmacology , Ion Channel Gating/drug effects , Membrane Potentials/drug effects , Membrane Potentials/genetics , Nerve Growth Factor/metabolism , Nerve Growth Factor/pharmacology , Neurons, Afferent/drug effects , Patch-Clamp Techniques , Rats , Rats, Sprague-Dawley , Signal Transduction/drug effects , Signal Transduction/genetics , Transcriptional Activation/drug effects , Transcriptional Activation/genetics , Up-Regulation/drug effects , Up-Regulation/geneticsABSTRACT
Cardiac contractile dysfunction is frequently reported in human patients and experimental animals with type-1 diabetes mellitus. The aim of this study was to investigate the voltage-dependence of contraction in ventricular myocytes from the streptozotocin (STZ)-induced diabetic rat. STZ-induced diabetes was characterised by hyperglycaemia and hypoinsulinaemia. Other characteristics included reduced body and heart weight and raised blood osmolarity. Isolated ventricular myocytes were patched in whole cell, voltage-clamp mode after correcting for membrane capacitance and series resistance. From a holding membrane potential of -40 mV, test pulses were applied at potentials between -30 and +50 mV in 10 mV increments. L-type Ca2+ current (I Ca,L) density and contraction were measured simultaneously using a video-edge detection system. Membrane capacitance was not significantly altered between control and STZ-induced diabetic myocytes. The I Ca,L density was significantly (p < 0.05) reduced throughout voltage ranges (-10 mV to +10 mV) in myocytes from STZ-treated rats compared to age-matched controls. Moreover, the amplitude of contraction was significantly reduced (p < 0.05) in myocytes from STZ-treated rats at all test potentials between -20 mV and +30 mV. However, in electrically field-stimulated (1 Hz) myocytes, the amplitude of contraction was not altered by STZ-treatment. It is suggested that in field-stimulated myocytes taken from STZ-induced diabetic hearts, prolonged action potential duration may promote increased Ca2+ influx via the sodium-calcium exchanger (NCX), which may compensate for a reduction in Ca2+ trigger through L-type-Ca2+-channels and lead to normalised contraction.
Subject(s)
Calcium Channels, L-Type/physiology , Diabetes Mellitus, Experimental/physiopathology , Heart Ventricles/physiopathology , Myocardial Contraction/physiology , Myocytes, Cardiac/physiology , Animals , Calcium/metabolism , Diabetes Mellitus, Experimental/chemically induced , Male , Rats , Rats, Wistar , Sodium-Calcium Exchanger/physiologyABSTRACT
This study compares the actions of the intravenous anaesthetics propofol and ketamine on animal behaviour and neuronal activity in the snail Lymnaea stagnalis, particularly in relation to excitatory effects observed clinically. When injected into the whole animal, neither agent induced total anaesthesia. Rather, behavioural activity was enhanced by propofol (10(-5) M) and ketamine (10(-7) M), indicating excitatory effects. When superfused over the isolated central nervous system (CNS), differential effects were produced in two identified neurons, right pedal dorsal 1 (RPeD1) and visceral dorsal 4 (VD4). Resting membrane properties were largely unaffected. However, spike after hyperpolarisation was significantly reduced in RPeD1, but not VD4, with some evidence of increased excitability. In addition, an intrinsic bursting property (post-stimulus burst) in VD4 was altered by propofol (10(-7) M). The results suggest significant excitatory components in the actions of some intravenous anaesthetics, as well as a potential role in modifying excitation and bursting mechanisms in the CNS.