AbstractHeart failure (HF) is the leading cause of mortality in the Western world and can belinked with a decrease in contractile function. Contraction in the heart is dependenton the synchronous rise of the systolic calcium (Ca) transient of individualmyocytes. In the healthy heart the presence of transverse tubules (t-tubules) enablesynchronous Ca release. Evidence shows t-tubule density varies between speciesand cell type and t-tubules are lost in several cardiac diseases. The consequence of ttubuleloss is decreased amplitude and heterogeneous rise of systolic Ca transientsand increased prevalence of arrhythmias. Despite this, the mechanisms controlling ttubuleformation and maintenance and if these factors differ between cell type and incardiac diseases remains unclear. Previous data points towards several proteinsinvolved in the biogenesis of t-tubules, including Amphiphysin (AMPII) andJunctophilin (JP2). It is therefore the aim of this thesis to determine, a role forAMPII and JP2 in controlling t-tubule formation in the heart and, the effect AMPIIhas on the systolic Ca transient.Atrial and ventricular tissue samples were collected from rat, ferret and sheep heartsand differences in t-tubule spacing / density assessed using di-4-ANEPPS or WGAstaining and confocal microscopy. AMPII and JP2 protein expression was assessedusing Western blotting. HF was induced by right ventricular tachypacing in sheepand by pressure overload of the left ventricle in ferret. T-tubule density and proteinexpression were assessed as described above. Small interfering RNA (siRNA)targeted against AMPII and JP2, in rat ventricular myocytes lead to proteinknockdown. AMPII siRNA treated myocytes were loaded with a Ca2+ indicator andelectrically stimulated. Confocal x-t line scans were obtained to assess the effect ofAMPII knockdown on the systolic Ca transient.T-tubule density and AMPII protein expression were lower in the atria than ventriclein all species studied. A reduction in t-tubule density was found in both the sheepand the ferret models of HF, which correlated with reduced AMPII protein.However, JP2 protein expression did not change in either model of HF. Knockdownof AMPII, but not JP2, lead to decreased t-tubule density. The data also showed aninverse correlation between AMPII protein levels and t-tubule spacing. AMPIIsiRNA knockdown mediated t-tubule loss also lead to delayed, dysynchronous andreduced systolic Ca transients.In conclusion, this study has shown that in cardiac muscle, AMPII is vital for ttubuleformation. Reduced AMPII expression could explain differences in t-tubuledensity between cells and species. Furthermore, the data suggests AMPII loss maybe partly responsible for t-tubule loss observed in HF. Additionally; knockdown ofAMPII provides a causal link between AMPII loss and t-tubule loss and alsosuggests that AMPII is required to maintain normal Ca signaling in the heart.