No abstract

Whole-cell voltage clamp was carried out on freshly dispersed single smooth muscle cells from adult rat and human colons to investigate the regulation of the Ca2+ channels. In this study, we unexpectedly discovered the existence of a fast Na+ channel current. With normal physiological salt solution (PSS) plus 4-amino-pyridine (3 mM) in the bath and high-Cs+ solution in the pipette to inhibit outward K+ currents, an inward current possessing fast and slow components was observed when the cell membrane was depolarized to a value more positive than -20 mV from a holding potential of -100 mV. When Ca2+ ions were removed from the PSS, or when nifedipine (10 microM) and Ni2+ (30 microM) were simultaneously applied, the slow component disappeared and the fast component remained. The fast current component became almost completely inactivated within 10 ms. This fast component was dependent on extracellular Na+ concentration and was inhibited by tetrodotoxin (TTX) dose dependently (IC50 of 130 nM in rat and 14 nM in human). These results suggest that the slow component of inward current was a Ca2+ channel current, whereas the fast component was a TTX-sensitive fast Na+ channel current. The threshold voltage, the voltage for peak current, and the reversal potential for the fast Na+ current were, respectively, about -50, -20, and +50 mV in rats, and -40, 0, and +60 mV in humans. The incidence of cells possessing fast Na+ currents depended on the region of the colon.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-gated Ca2+ currents were investigated in single smooth muscle cells freshly isolated from the circular layer of the human colon (ascending and descending portions) using the whole cell voltage-clamp technique. Tissue samples were obtained at the time of therapeutic surgery. In physiological salt solution (containing 2 mM Ca2+), an inward current was observed when the cell membrane was depolarized in the presence of tetrodotoxin. This current disappeared when Ca2+ was removed from the bath solution and was inhibited when Ca2+ channel blockers were applied, indicating that the inward current was a Ca2+ current (ICa). Changing the holding potential (HP) from -100 mV to more positive potentials (e.g., -60 and -40 mV) markedly decreased the amplitude of ICa. The voltage dependence of steady-state activation and inactivation was represented by Boltzmann distributions; there was a substantial amount of overlap (window current) between -60 and -10 mV. A fast-inactivating ICa component followed by a slow-inactivating ICa component was observed in some cells from both ascending and descending colons. The fast ICa component was observed only when cells were held at -80 or -100 mV, and had a more negative threshold potential (-70 to -60 mV). This component was sensitive to low concentrations of Ni2+ (30 microM) but was resistant to nifedipine (10-20 microM). In contrast, the slow (sustained) ICa component was observed at all HPs (-40 to -100 mV) and had a more positive threshold potential (about -40 mV). This component was insensitive to low concentration of Ni2+ but was sensitive to nifedipine and BAY K 8644.(ABSTRACT TRUNCATED AT 250 WORDS)

The slow Ca2+ channels (L-type) of the heart are stimulated by cAMP. Elevation of cAMP produces a very rapid increase in number of slow channels available for voltage activation during excitation. The probability of a Ca2+ channel opening and the mean open time of the channel are increased. Therefore, any agent that increases the cAMP level of the myocardial cell will tend to potentiate ICa, Ca2+ influx, and contraction. The action of cAMP is mediated by PK-A and phosphorylation of the slow Ca2+ channel protein or an associated regulatory protein (stimulatory type). The myocardial slow Ca2+ channels are also regulated by cGMP, in a manner that is opposite or antagonistic to that of cAMP. We have demonstrated this at both the macroscopic level (whole-cell voltage clamp) and the single-channel level. The effect of cGMP is mediated by PK-G and phosphorylation of a protein, as for example, a regulatory protein (inhibitory-type) associated with the Ca2+ channel. Introduction of PK-G intracellularly causes a relatively rapid inhibition of ICa(L) in both chick and rat heart cells. Such inhibition occurs for both the basal and stimulated ICa(L). In addition, the cGMP/PK-G system was reported to stimulate a phosphatase that dephosphorylates the Ca2+ channel. In addition to the slower indirect pathway--exerted via cAMP/PK-A--there is a faster more-direct pathway for ICa(L) stimulation by the beta-adrenergic receptor. This latter pathway involves direct modulation of the channel activity by the alpha subunit (alpha s*) of the Gs-protein. In vascular smooth muscle cells the two pathways (direct and indirect) also appear to be present, although the indirect pathway produces inhibition of ICa(L). PK-C and calmodulin-PK also may play roles in regulation of the myocardial slow Ca2+ channels. Both of these protein kinases stimulate the activity of these channels. Thus, it appears that the slow Ca2+ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of factors intrinsic and extrinsic to the cell, and thereby control can be exercised over the force of contraction of the heart.

Increased excitability of primary sensory neurons may be important for the generation of neuropathic pain from nerve injury. The currents underlying the action potentials of these neurons are largely carried by Na+, and changes in Na+ currents have been postulated to contribute to this increased excitability. Using patch clamp in whole-cell mode, we recorded Na+ currents from DRG neurons freshly isolated from rats with a chronic constriction injury (CCI), an animal model of neuropathic pain. We found significant changes in Na+ currents after CCI when cell size and Na+ channel properties were used to segregate DRG neurons. Most changes were concentrated in small neurons (< or = 25 microm diameter) and in the slow TTX-resistant current that is predominant in these cells. CCI produced two principal changes in these cells: it shifted the voltage-dependence of activation of the TTX-resistant current to more negative potentials and it reduced the average density of this current. The decrease in density appears to be primarily due to the decrease in the number of small neurons expressing this current. The net result is a change in both activation and steady-state inactivation properties of the total Na+ current to more negative potentials without a significant change in the density of total Na+ current. The change in activation properties of the TTX-resistant Na+ current are similar to those produced by some hyperalgesic autacoids, and may contribute to the increase in primary afferent excitability and hyperalgesia that occurs after this lesion.

No abstract

This case demonstrates the need to consider lipid emulsion therapy in the advanced cardiac life support algorithm for lidocaine toxicity as well as other lipid soluble drug intoxications.

Epidural analgesia may be safely used in patients undergoing major hepatic resection, providing that they have normal pre-operative coagulation and catheters are removed only when resection-induced perioperative coagulopathy has resolved or has been corrected.