The present invention discloses a peptide of SEQ ID NO:2 and its variants that blocks stretch-activated ion channels. The peptide, designated as GsMTx-4, is present in the venom of the spider Grammostola spatulata. The present invention also discloses a method of purifying the peptide GsMTx-4 from the spider venom and a method for inhibition of stretch activated ion channels in a cell. The cDNA sequence encoding the GsMTx-4 is also disclosed. This peptide and its variants can be used for the treatment of cardiac arrhythmias.