Pediatric Nutrition in Practice

58 Prince  Fuchs

Intracellular Regulators of Ion Flux

Various hormones, neurotransmitters, and secre-
tagogues bind to receptors along the epithelial cell
membrane to initiate the intracellular cascade in-
volving second messenger molecules of cyclic nu-

cleotides (including cyclic adenosine monophos-

phate and cyclic guanosine monophosphate) and

ionized cytosolic calcium (Ca 2+ ). These in turn

activate protein kinases that exert direct control

of ion channels to increase the efflux of Cl through

Cl channels down their electrochemical gradients

and the inhibition of electroneutral NaCl-cou-

pled influx.

Intercellular Regulators of Ion Flux

Under normal conditions, the intestinal transport

of water and electrolytes is a finely tuned transcel-

lular and paracellular phenomenon regulated by

complex interactions between the endocrine,

paracrine, immune, and enteric nervous systems.

In reality, these systems do not function as iso-

lated units; their borders are indistinct and over-

lap [4]. Examples include serotonin and vasoac-

tive intestinal peptide, which function as hor-

mones or neurotransmitters or both depending

on the precise physiologic situation. Certain bac-

terial enterotoxins such as cholera and cytotoxins

simultaneously stimulate paracrine, neural, and

immune responses, all of which may alter ion and

water flux [7].

Glucose

Glucose

3Na+

2Na+ 2K+

Cl–

SGLT-1 GLUT-2

Na+/K+-

ATPase

3Na+

Na+

2K+

K+

2Cl–

K+

Na+

Cl–

CFTR NKCC1

Na+/K+-

ATPase

Fig. 1. Cotransportation of sodium and glucose. Fig. 2. Chloride excretion and fluid secretion.

3.0

2.5

2.0

1.5

1.0

0.5

0

160 200 240 280

Cool and dry

Hot and humid

8 7 6

Running speed (min/mile)

Running speed (m/min)

10 9 5

320

Sweat rate (l/h)

Fig. 3. The effect of ambient temperature, humidity, and
exertion (running speed) on the approximate rate of
sweating in adults. From Sawka and Montain [8].

Koletzko B, et al. (eds): Pediatric Nutrition in Practice. World Rev Nutr Diet. Basel, Karger, 2015, vol 113, pp 56–61

DOI: 10.1159/000367869