salamz
..any1 plz ... i read somewhere that it weakens the heart activity..n at other place i read it increases the heart rate ..i annot understand ..can plz any1 explain ..with proper logic ..thanx

Low levels of K cause increased activity (which may lead to an irregular heartbeat), whereas high levels cause decreased activity. Either situation can lead to cardiac arrest in some scenarios.
Most cases of hyperkalaemia are related to the kidneys' inability to excrete potassium.

A simple way of understanding this is to remember that Na enters the cell (depolarisation) and K leaves (repolarisation). If there's high extracellular K, then less intracellular K leaves. This is known as 'contracture'.The cells are unable to maintain a sustained contraction (depolarisation) and so becomes fatigued, hence decreased heart activity (reduced action potential duration, slowed conduction, decreased pacemaker rate, and decreased pacemaker arrhythmogenesis).

With low extracellular K (N.B. THIS IS MY WAY OF REMEMBERING THIS-IT IS NOT A PROVEN FACT NOR POSSIBLE !!!!!) SO DON'T GO QUOTING ME
yes, back to the topic, with less extrcellular K, more K leaves the cell, hence there is a faster cardiac cycle with prolonged action potential duration, increased pacemaker rate, and increased pacemaker arrhythmogenesis.

Longstanding hypokalemia causes rhabdomyolysis via a direct effect on muscle cell metabolism and probably through muscle ischemia (it blocks the vasodilation that normally occurs during exercise).