Hi, what do you mean? by substances? chracteristic subtances for each type of tissues? or what exactly? if you mean that, i can say yes but you have to bring in mind that tissues secretion depend on the physilogical status of the tissues, stumilators, some substances are not stable, so you have to clarify more to can help you.

I am doing Realtime PCR, and the samples are from different tissues (heart, kidney, liver, lung.....), how to choose reference gene that stable expressed in different tissues, as actin or GAPDH that usually used in Realtime PCR.

I would look up 'genorm' (or bestkeeper or normfinder).  You would run a PCR with samples from each of your tissues with a panel of housekeeping genes and then choose the most stable genes.  It is recommended that you use at least 3 reference or housekeeping genes for normalisation of qRT-PCR as no housekeeping gene is completely stable, especially when you are analysing expression across different tissues.  Primer Design are a small company who can supply Genorm kits (for SYBR green or taqman)  but you can also pick your own panel of genes and design your own primers of course.