Which may be the most dangerous thing in electronic cigar. what is the possible effect of it to kidneys?

Nicotine: Electronic cigs have nicotine in them so they are still problematic as far as side effects of nicotine including high blood pressure, vessel injury, accelerated atherosclerosis and salt retention. They also have some chemicals in the sme that is inhaled and the side effects have not been adequately studied at this point. They likely accelerate loss of kidney function via vascular injury.

Nicotine.: Excellent response by Dr. Gitomer. E- cigarettes may contain nicotine, volatile organic compounds, heavy metals (like lead, tin & nickel), ultrafine particles (which may be drawn deep into lungs, diacetyl & Nicotine which is addictive. REF: https://e-cigarettes.surgeongeneral.gov/. Agree with earlier comment regarding renal function.