Slow inactivation of a tetrodotoxin-sensitive current in canine cardiac Purkinje fibers.

We used the two-microelectrode voltage clamp technique and tetrodotoxin (TTX) to investigate the possible occurrence of slow inactivation of sodium channels in canine cardiac Purkinje fibers under physiologic conditions. The increase in net outward current during prolonged (5-20 s) step depolarizations (range -70 to +5 mV) following the application of TTX is time dependent, being maximal immediately following depolarization, and declining thereafter towards a steady value. To eliminate the possibility that this time-dependent current was due to inadequate voltage control of these multicellular preparations early during square clamp pulses, we also used slowly depolarizing voltage clamp ramps (range 5-100 mV/s) to ensure control of membrane potential. TTX-sensitive current also was observed with these voltage ramps; the time dependence of this current was demonstrated by the reduction of the peak current magnitude as the ramp speed was reduced. Reducing the holding potential within the voltage range of sodium channel inactivation also decreased the TTX-sensitive current observed with identical speed ramps. These results suggest that the TTX-sensitive time-dependent current is a direct measure of slow inactivation of canine cardiac sodium channels. This current may play an important role in modulating the action potential duration

Gating of delayed rectification in acutely isolated canine cardiac Purkinje myocytes. Evidence for a single voltage-gated conductance.

Studies of time-dependent, plateau outward current (delayed rectification) in the heart are complicated by the accumulation and depletion of K+ ions in intercellular clefts. To minimize this problem, we studied delayed rectification in acutely isolated (enzymic solution, gentle agitation) canine cardiac Purkinje myocytes using the single microelectrode voltage-clamp technique. We found a sigmoidal voltage-dependence for activation of outward plateau current, with maximal activation occurring at potentials near -10 mV. The activation and deactivation of plateau outward current was adequately described as the sum of a fast and slow exponential component. A comparison of the time course of activation of plateau outward current and the "envelope" of tail currents suggests that a single voltage-gated conductance with one open and two closed states can account for delayed rectification in Purkinje myocytes. These results differ from those previously obtained with intact sheep Purkinje fibers in which two time-dependent conductances were postulated to account for delayed rectification (Noble, D., and R. W. Tsien, 1969, J. Physiol. (Lond.), 200:205-231)

Small vessel childhood primary angiitis of the central nervous system with positive anti-glial fibrillary acidic protein antibodies: a case report and review of literature

Abstract Background Small vessel childhood primary angiitis of the central nervous system (SV-cPACNS) is a rare disease characterized by inflammation within small vessels such as arterioles or capillaries. Case presentation We report a case of SV-cPACNS in an 8-year-old boy confirmed by brain biopsy. This patient was also incidentally found to have anti-glial fibrillary acidic protein (GFAP) antibodies in the cerebrospinal fluid (CSF) but had no evidence of antibody-mediated disease on brain biopsy. A literature review highlighted the rarity of SV-cPACNS and found no prior reports of CSF GFAP-associated SV-cPACNS in the pediatric age group. Conclusion We present the first case of biopsy proven SV-cPACNS vasculitis associated with an incidental finding of CSF GFAP antibodies. The GFAP antibodies are likely a clinically insignificant bystander in this case and possibly in other diseases with CNS inflammation. Further research is needed to determine the clinical significance of newer CSF autoantibodies such as anti-GFAP before they are used for medical decision-making in pediatrics

Can the Ca(2+) hypothesis and the Ca(2+)-voltage hypothesis for neurotransmitter release be reconciled?

It is well established that Ca(2+) plays a key role in promoting the physiological depolarization-induced release (DIR) of neurotransmitters from nerve terminals (Ca(2+) hypothesis). Yet, evidence has accumulated for the Ca(2+)-voltage hypothesis, which states that not only is Ca(2+) required, but membrane potential as such also plays a pivotal role in promoting DIR. An essential aspect of the Ca(2+)-voltage hypothesis is that it is depolarization that is responsible for the initiation of release. This assertion seems to be contradicted by recent experiments wherein release was triggered by high concentrations of intracellular Ca(2+) in the absence of depolarization [calcium-induced release (CIR)]. Here we show that there is no contradiction between CIR and the Ca(2+)-voltage hypothesis. Rather, CIR can be looked at as a manifestation of spontaneous release under conditions of high intracellular Ca(2+) concentration. Spontaneous release in turn is governed by a subset of the molecular scheme for DIR, under conditions of no depolarization. Prevailing estimates for the intracellular calcium concentration, [Ca(2+)](i), in physiological DIR rely on experiments under conditions of CIR. Our theory suggests that these estimates are too high, because depolarization is absent in these experiments and [Ca(2+)](i) is held at high levels for an extended period