Myosin phosphatase is a trimeric protein that regulates vascular tone by inducing dephosphorylation of smooth muscle regulatory myosin light chains. The 110-130 kDa myosin phosphatase targeting subunit (MPYT) is essential for this process. Nitric oxide- mediated arterial relaxation has been suggested to occur partly via PKG interactions with a leucine zipper (LZ) region of MYPT1 that is generated by alternative exon splicing of the 3′ end of the gene [1]. The relative expression of MYPT1 LZ+ to LZ- isovariants may thus influence arterial sensitivity to endothelial-mediated vasodilators and/or NO donors. Interestingly, the sensitivity of isolated human myometrial arteries to endothelial-mediated vasodilators is greater than that of placental arteries from pregnant women (2). In addition, non-vascular uterine smooth muscle shows variable responsiveness to NO donors. We propose that tissue-specific differences in human MYPT1LZ+ to LZ- isovariant expression, not previously assessed in human tissues, may offer an explanation for these findings. Human tissue samples were obtained, following written informed consent, from normal pregnant women undergoing elective Caesarean section at term. Myometrium, myometrial arteries (MA) and placental arteries (PA) were microdissected free of all surrounding tissue. The human MYPT1LZ+ sequence (NM_002480.1) was aligned with chicken (NM_205123) and mouse (NM_027892) MYPT1LZ- sequences to detect a shift in the reading frame that generates the putative human MYPT1LZ- sequence. RNA was isolated (Qiagen RNeasy fibrous kit) and reverse transcribed (Stratagene AffinityScript cDNA synthesis Kit) from the human tissues and qPCR, with isovariant-specific primers and PerfectProbe amplification (Primer Design, UK), used to measure MYPT1LZ+ and MYPT1LZ- isovariant expression. Human smooth muscle RNA (Clonetech, USA) was used as an internal calibrator. MYPT1LZ+ expression was less in the myometrium (fold-change relative to calibrator RNA of 0.33+0.04) than MA (1.33+0.12) and PA (2.07+0.17; p<0.01, n=10, mean+SEM, ANOVA). A similar pattern was seen for MYPT1LZ- expression wherein changes relative to calibrator were 0.06+0.17 (myometrium), 0.45+0.05 (MA) and 0.65+0.07 (PA). This reveals differences in MYPT1LZ+ and MYPT1LZ- expression between human myometrium, MA and PA. Of note, the ratio of MYPT1LZ+ to LZ- expression in MA was not greater than in PA. As the former is more sensitive to endothelial-dependent relaxant agonists, this suggests that a correlation between MYPT1LZ+ expression and physiological sensitivity to PKG and/or NO stimulation in these human tissues may be an over-simplification. Further research is required, however, to determine such a relationship in relevant clinical conditions of heightened smooth muscle tone.