In both cases, it forms no part of the circuit of industrial capital.
As Marx sets out in Capital II and III, this loanable money-capital/interest-bearing capital, sits outside the circuit of industrial capital. It is why this fictitious-capital play no part in the determination of the average industrial rate of profit, and obtains not this average rate of profit (or, indeed, any “profit”), but only interest as a deduction from profit. Far from shareholders being just a continuation of the old, private industrial capitalists, they are, for this very reason, as Marx describes, in Capital III, antagonistic to the interests of industrial capital, and particularly, the current, collective owners of socialised, industrial capital, i.e. “the associated producers”.
Where the private industrial capitalists were antagonistic to workers, as workers, because higher relative wages means lower relative profits, the owners of fictitious-capital (shares, bonds) are antagonistic to workers, not as workers, but as, objectively, the owners of socialised industrial capital. The ruling-class, as owners of shares and bonds, seek to maximise the amount they get as interest/dividends, just as landlords seek to maximise the amount they get as rent, but that means the smaller the amount of profit retained for capital accumulation, i.e. profit of enterprise. It is why the ruling class seeks to retain control over that socialised capital, so that it maximises its revenues – not, now, profit but interest/dividends and capital gains – and appoints Directors to that end.
The workers – associated producers – as collective owners of the socialised capital, as Marx notes in Capital III, Chapter 27, resolve the contradiction between capital and labour by becoming their own capitalist. That is most clearly seen in the worker cooperative. In the worker cooperative, the workers exercise democratic control over that capital, and they also appoint their own day to day managers to carry out the role of “functioning capitalist”, who, to use Marx's description is like an orchestra conductor.
But, even in the worker cooperative, the means of production are still capital, and must remain so as long as commodity production continues to determine the nature of the economy. Each company/cooperative continues to produce commodities for sale, and so competes against other commodity producers. It is exchange value that determines production, not use-value. In order to be competitive, each company must keep the individual value of what it produces below the market value.
It does that by all the same means that every other capital does. Wages cannot rise above the value of labour-power, for example. If they do, then, the rate of surplus value falls. The firms profit falls below the average profit. Over time, this lower level of profit means the cooperative cannot accumulate the capital required to expand production; it loses market share.
The other way a capital stays competitive, even if it does pay higher wages, is precisely this accumulation of capital. In Capital I, Marx notes that, in the 19th century, although British textile workers' wages were 50% higher than those in Europe, British textiles were always cheaper than those produced in Europe, and British profits were also higher. The reason was that the larger scale of production, in Britain, the greater number of machines, and more advanced nature of those machines, meant that, even with higher wages, unit labour costs, and also, unit fixed capital costs, were much lower.
But, in order to accumulate that capital in the first place, it is necessary to maximise profit, so as to be able to buy more and better fixed capital. In a capitalist economy, a worker cooperative is still bound by these laws of capital. The difference for the cooperative is that, in seeking to maximise its rate of surplus value/exploitation, it does so for this purpose of being able to accumulate capital, and so ensure its competitiveness, and longer-term future. Soviet Russia faced the same situation in 1917.

