Myocardial potassium uptake during alpha- and beta-adrenoceptor stimulation

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Alpha 1-adrenoceptor subtypes involved in increased 86Rb+ influx rate and inositol 1,4,5-trisphosphate mass in adult rat cardiomyocytes

The aim of the present study was to identify the receptor subtypes involved in the alpha 1-adrenoceptor-mediated increase in 86Rb+ influx rate and in inositol 1,4,5-trisphosphate (IP3) accumulation in isolated ventricular cardiomyocytes from adult rat heart, in order to identify a possible response pattern compatible with a causative relationship. Subtype-selective receptor antagonists used were: 5-methylurapidil (alpha 1A), WB 4101 [2([2,6-dimethoxyphenoxy-ethyl]aminomethyl)-1,4-benzodioxane] (alpha 1A), chloroethylclonidine (alpha 1B) and BMY 7378 [8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]dec ane-7,9-dione] (alpha 1D). The basal 86Rb+ influx rate was 0.22 +/- 0.01 ml/g protein x min. At 15 min, 5 x 10(-5) mol/l noradrenaline in the presence of 3 x 10(-5) mol/l timolol increased the 86Rb+ influx rate by 33 +/- 1%. This response was not affected by either chloroethylclonidine or BMY 7378 at concentrations up to 10(-5) mol/l. 5-Methylurapidil dose dependently inhibited the response to 5 x 10(-5) mol/l noradrenaline with a -logIC50 value of 5.27 +/- 0.12 and 5.61 +/- 0.27 in the presence and absence of 10(-5) mol/l chloroethylclonidine, respectively. WB 4101 in the presence of 10(-5) mol/l chloroethylclonidine dose dependently inhibited the response to 5 x 10(-5) mol/l noradrenaline with a -logIC50 value of 6.10 +/- 0.14. Noradrenaline in the presence of 10(-5) mol/l chloroethylclonidine dose dependently increased the 86Rb+ uptake rate with a -logEC50 value of 6.19 +/- 0.35. The basal IP3 level was 2.15 +/- 0.19 pmol/mg protein. Incubation with 10(-5) mol/l noradrenaline for 2 min increased this by 65 +/- 7% of control levels. 10(-5) mol/l chloroethylclonidine and 10(-4) mol/l 5-methylurapidil reduced the response to 27 +/- 6% and 18 +/- 9% of control level, respectively. BMY 7378 dose dependently inhibited the IP3 response at relatively high concentrations, and it was completely eliminated at 10(-5) mol/l BMY 7378. The combination of chloroethylclonidine and 5-methylurapidil or 3 x 10(-6) mol/l prazosin alone completely abolished the hormone-induced effect. We conclude that whereas the alpha 1-adrenoceptor-stimulated increase in 86Rb+ influx rate is mediated via the alpha 1A-adrenoceptor subtype only, both alpha 1A- and alpha 1B-adrenoceptor subtypes are involved in the increase in IP3 mass. Furthermore, a contribution from the alpha 1D-adrenoceptor in the IP3 response cannot be excluded. Thus there does not appear to be a simple causative relationship between an increase in 86Rb+ influx rate and an increase in IP3.