Aspectos generales

Inward rectifier potassium channels act like diodes which favour an inward flow of potassium ions during hyperpolarisation and very little ion flow during depolarisation. For cells that express predominantly inward rectifier chaimels, extended periods o f depolarisation can be maintained, which is important for the generation o f long action potentials. The small outward conductance o f the inward rectifier channels at membrane potentials just above the potassium reversal potential (EpJ, which is about - 90mV, helps to maintain the resting membrane potentials close to Ek- Thus the inward rectifier channels play a role in maintaining neurone resting membrane potential and in controlling excitability. A typical inwardly rectifying potassium current can be seen in figure 3.22b.

Figure 3.22c shows the mean normalised amplitude o f the inwardly rectifying potassium current generated from a 900msec hyperpolarising step to -120mV, from a

holding potential of -70mV. There was no significant difference in the current amplitudes of the inwardly rectifying potassium current of uninfected or wt HSV 1T

infected DRG neurones. The presence of the inwardly rectifying potassium currents were also observed by clamping the neurones in the current clamp mode and eliciting a family of voltage traces with a series of hyperpolarising current steps. When inward rectifier currents were expressed in DRG neurones the characteristic time-dependent rectification was observed, as in figure 3.22a.

a InA 20mV 1 0 0ms 0.5nA 1 0 0ms

0.000 ^ -0.005 ^ pu D. < 5 -0.010 - 0 .0 1 5 - -0.020 - -0.025 -

con trol W tH S V in fected

n = 7

F igu re 3.22. A com parison o f the inw ardly rectifying, p otassium cu rren t am p litud e, n orm alised for neurone size o f control or w t H SV 17+ infected D R G neurones, a A family of voltage traces, elicited by 900ms hyperpolarising current steps from a wt HSV 17+ infected DRG neurone, showing time dependent rectification, b An inwardly rectifying potassium current evoked from a wt HSV 17+ infected DRG neurone, following a step from -70mV to -120ms, for 900ms. c The graph shows the mean normalised sustained current amplitude ± s.e.m of control or wt HSV 17+ infected DRG neurones. There was no significant difference between the normalised current amplitudes (Student t test, P>0.1).

A family o f inwardly rectifying potassium currents were evoked from a holding potentials o f -70mV, with step depolarisations from -50mV to -140mV. The tail current amplitudes were measured upon membrane repolarisation to -70mV, normalised with respect to peak tail current and then plotted against command potential. The activation curve is illustrated in figure 3.23, and shows the activation curves o f wt HSV 17^ infected and uninfected DRG neurones almost overlap. This indicates no obvious change in the activation o f inwardly rectifying potassium chaimels during a wt HSV 17^ infection. The electrophysiological characteristics, calculated from such curves fitted with a Boltzmann function for each neurone tested.

are shown in table 3.9. Statistical analysis showed that there was no significant difference in the slope factor and V5 0 values.

w t H S V i n f e c t e d

0.6- 0 . 4 - 0.2- - 1 2 0 -100 - 1 4 0 -80 -60 command potential (mV) - 4 0 -20

Figure 3.23. Activation curves for the hyperpolarisation activated conductances recorded from control and HSV infected DRG neurones, a A fa m ily o f in w a rd ly r e c tifin g p o ta s s iu m cu rren ts e v o k e d b y a ra n g e o f p o te n tia ls b e t w e e n - 5 OmV and - 1 4 0 m V .

b T h e n o r m a lise d m ea n inw ard tail cu rrent a m p litu d e ± s .e .m reco rd ed o n rep o la r isa tio n

to th e h o ld in g p o te n tia l, p lo tted a g a in s t th e h y p e r p o la r isin g c o m m a n d p o te n tia l. T h e

a c tiv a tio n c u r v e o f c o n tr o l ( • ) n = 9 , and w t H S V M ' ^ in fe c te d ( ■ ) n = 7 D R G n e u r o n e s are

s h o w n . T h e a c tiv a tio n c u r v e s s h o w n are o b ta in ed b y fittin g th e m ea n n o r m a lise d data to a B o ltz m a n n fu n c tio n .

V50 slope factor number o f cells (mV) (mV)

control -94.1 ± 1.4 10.6± 1.8 9 WtHSV 17^ -91.2 ± 1.6 8.7 ± 0.7 7

Table 3.9. The slope factor and V5 0 from the activation curves of

hyperpolarisation activated conductance in control or wt H SV 17"^ infected DRG neurones. T h ere w a s n o s ig n ific a n t d if fe r e n c e b e tw e e n th e V 5 0 and s lo p e fa c to r v a lu e s o f th e in w a r d ly r e c tify in g p o ta s siu m c h a n n e l a c tiv a tio n c u r v e s (S t u d e n t s ’s t te s t, P > 0 .0 8 ).

Inwardly rectifying potassium channels are not uniformly distributed throughout a population of DRG neurones, and are predominantly found in the larger DRG neurones. The distribution of the inwardly rectifying potassium currents of DRG neurones was not altered during a wt HSV \ Ÿ infection, as shown in figures 3.24 and 3.25. 5 - .2 4 - 2- I 1 no Ik I Ilk

I

10 20 30 40 50 60 70 capacitance (pF) 80 90 100

Figure 3.24. The differential distribution o f inwardly rectifying potassium current expression with respect to capacitance in control neurones. The \\\ shading indicates those cells expressing inwardly rectifying potassium current, while those that did not express a current are indicated by ||| shading. The total number o f cells tested was 38, bin size 10 pF.

6 - 5 -

I -

O 3 0 1 2 - I'': I ~1 no Ik I- - - - 1 I k 10 20 30 40 50 60 70 capacitance (pF) 80 90 100

Figure 3.25. The differential distribution o f inwardly rectifying potassium current expression with respect to HSV infected cell capacitance. The \\\ shading indicates those cells expressing inwardly rectifying potassium current, while those that did not express a current are indicated by ||| shading. The total number o f cells tested was 26, bin size 10 pF.

In summary, the outward, delayed rectifier type potassium conductances were unaltered 24 hours after wt HSV 17^ infection o f DRG neurones. The inwardly rectifying potassium current was also unaffected by the infection. This is in contrast to the sodium conductance which was dramatically reduced in DRG neurones which had also been infected with wt HSV 17^. wt HSV 17^ infection o f DRG neurones specifically causes a loss in sodium conductance and not other ionic currents.

In document Continuos Homogéneos con respecto a conjuntos denso numerables (página 64-71)