It should be emphasized that many aspects of the four general feedback transmission steps noted earlier remain unclear or controversial. Nevertheless, recent data have provided sufficient insight to allow at least a basic formulation of some of the cellular processes involved in the transmission of feedback signals. Figure 6 is a composite electron micrograph representation of the macula densa juxtaglomerular complex. We are proposing that calcium may serve two distinct roles in the transmission of feedback signals. First, in response to an increase in luminal fluid concentration there is an activation of cytoplasmic calcium within the macula densa feedback receptor cells. The increase in cytosol calcium in turn leads to the transmission of a signal to the glomerular vascular elements (step 3). Finally, the second role for calcium occurs at the vascular beds where calcium is probably responsible for excitation-contraction coupling of the glomerular contractile elements which could include both mesangial structures as well as arteriolar smooth muscle cells.